In my current campaign (in which I am a player) we are considering switching over to the Gritty Realism rest variant in the DMG so that the party would have to expend more resources between resting. The variant says:

This variant uses a short rest of 8 hours and a long rest of 7 days.

But we are confused at how this variant interacts with the normal rules for resting.

A normal long rest has this description:

A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch. If the rest is interrupted by a period of strenuous activity - at least 1 hour of walking, fighting, casting spells, or similar adventuring activity - the characters must begin the rest again to gain any benefit from it.

How do these rules transfer over to the variant? Is it true that you must spend 7 full days only in a single long rest and that you may not perform more than 2 hours of light activities over the entire 7 day (168 hour) period? It seems like over 7 days you could easily spend at least 2 hours eating and talking.

Would the long rest be invalidated if they take are interrupted by a single hour of strenuous activity over the entire 7 days? Even for a "gritty" variant that seems really extreme.

How are the activity limitations supposed to work in this variant?

There is no official interpretation

The Gritty Realism rules seem to be an afterthought, slapped on top of the usual rules.

At least the 2 hours of light activities per long rest are hard to implement, I would go crazy with 166 hours of bed rest.

How we do it

Short Rest

This one is easy, just replace "1" with "8" in the description of short rests:

A short rest is a period of downtime, at least 8 hour long, during which a character does nothing more strenuous than eating, drinking, reading, and tending to wounds.

Long Rest

The 1 hour strenuous activity limit is per day. The implied 6 hours of sleep become an explicit daily minimum.

If you fail to comply, that day is lost, but you do not have to start the 7 days again.*

*) otherwise 61 minutes of running away every 167 hours could slowly kill you

Your interpretations are correct (at least by RAW, My RAI beliefs are at the bottom).

"Gritty Realism" long rests are easily interrupted

Nothing about the option changes the limitations for what interrupts resting. "Gritty Realism" is, honestly, a bad name; I don't have a better one but I'm also not a publisher that gets paid to name things. "Life is hard and people don't heal so great after fighting monsters" isn't the best alternative name, but you get the idea.

Let's look at another part of that section:

It's a good option for campaigns that emphasize intrigue, politics, and interactions among other NPCs .... [snip] ... combat is rare ... [snip]

I think most of us agree that this option isn't fleshed out, but even so; it's not meant to make dungeons harder, it's meant to make them all but nonviable.

You (and your DM) likely want a different option

Two options from the DMG are:

Healers Kit Dependency (no hit-die healing without healers kit)

Slow Natural Healing (no auto-healing after long rests)

If you put those two things together, you have an inherent resource drain. Someone (or numerous someones) has to stock up on healing kits (money and weight, if you track those sorts of things) and you're probably blowing 1-2 spells per day on just being fresh in the morning.

There is also the Lingering Injuries system, though I don't recommend using the permanent options. I would stick to the short-term lingering injuries, such as broken rib or festering wound.

If you're bent on using something like the Gritty Realism option...

I believe that RAI for this option is to have an 8-hour short rest every day and to not have more than 1 hour of "strenuous activity" per week.

I would, personally, houserule one of these two things:

• You count off "short rests" (8 hours of sleep or whatever), and every seven of those, you get to redeem them for a long rest. Like a free sub at the sandwich shop.

• Reportion all of the values; 7 nights of 6 hours of sleep; 7 days of up to an hour of "strenuous activity"; and the rest of the time counts as "light activity". This would change your long rest to:

A long rest is a period of extended downtime, at least 7 days long, during which a character sleeps for at least 42 hours and performs no more than 126 hours of light activity, such as reading, talking, eating, or standing watch. If the rest is interrupted by a period of strenuous activity - at least 7 hours of walking, fighting, casting spells, or similar adventuring activity - the characters must begin the rest again to gain any benefit from it.

Maybe your group would prefer more strenuous activity (perhaps 16 hours) or needs more sleep. Your mileage may vary, but the concept remains the same.

Interpretation depends on the DM (or the group).

I would simply add "per day" in the definition of a long rest.

A long rest is 7 days long. At least 6 hours of sleep per day. No more than 1 hour of adventuring per day.

No more than 6 hours of light activity per day. The two hours are multiplied by 3 to account for having three 8-hour periods in a day-night cycle.

I would also allow downtime activities (studying, researching, sparring) but limit it to 6 hours per day.

You will notice that there are 6 hours missing. They can be distributed to sleep/light activities/downtime, or the players can come up with ideas.

It's called "gritty"! It’s supposed to be tough and uncompromising. So I would assume you're treating it as the description of a long rest states, but you're doing it each day for seven days.