Depending on campaign style the Warlock player may be able to purposefully influence the number of short rests. Taking cooks tools, specializing in soups that require simmering, and RPing meal time is a great way to ensure that mealtimes are extended enough to count as rests, while simultaneously adding color and fun to the game.
Well, as a baseline, I would expect short rests to include mealtimes so a lunch and dinner would be two times a short rest would be expected. There are some logistical concerns regarding local hostiles, but if that's addressed you're fine.
Or a loremaster might skip the usual post-dinner campfire social activities to read, and reading explicitly qualifies for a short rest.
If you are using an ability then you are not resting, by definition. If they're just reading then what does it matter?
Or for a more mechanical option, being an Elf gives you four free short rests while everyone else finishes their long rest. Either approach is good for reliably getting extra castings of spammable spells.
If you are using an ability then you are not resting, by definition. If they're looking for 4-8 castings of a spell overnight, fine, but then it's a time span greater than four hours.
For example, an Elf Great Old One Warlock can cast Sending 8 times to check in with contacts while the party finishes their short rest, helping the party keep much better informed of ongoing events. (Of course, depending on playstyle keeping up on ongoing events can range from a game-changing strategic strength to utterly unhelpful, so your mileage may vary.)...
In other words, I don't think it's possible to establish a reasonable expected baseline. Too much will vary by campaign style, including how player-driven the campaign is, whether rests are incidental or require declaration, the use of doomclocks, and even hour often mealtime is a thing and not glossed over.
Okay, assuming you meant "long rest" in the example above, I could see that. It would take a total of 5 hours or so in my game, but I see your point.
I think it is possible to establish a baseline, not that hard, and indeed necessary. I think that it is important for the players and DM to have an understanding of the capabilities of the classes. In order to do that you need to have an understanding of time and costs in the campaign. Power and capabilities must come at a definable cost, otherwise you have a situation that can be absurdly exploited.
Is 59 minutes enough for a short rest? (Casting the spell(s) and the resting for the balance)
Is 61 minutes close enough to 60 that we're not going to care?
Are we really going to keep track of minutes? (Of course not)
Does the warlock, and every other class that relies on short rests, having access to about 16 power refreshes during the day appropriate to the balance of the other classes?
If not, what would be a good number? 12? 8? 4?
These are decisions that we are capable of reaching. We might not all agree on the exact number, but a concordance is not unreasonable. And, if after a few sessions it turns out to be untenable then adjust the number after a discussion at the table. The point of the warlock, it seems to me, is that the class has few active options but can mini-nova sporadically. While you're not going to get a 1:1 balance with other classes, we are capable to
choose a point of balance and then adjust as appropriate.
Looking at the rules and wringing our hands in indecision is not helpful. (Not directed specifically at you, Xetheral, but in general.)
