As you may know, some games have resources, such as mana, that regenerate over time. (Note that this discussion could apply to any resource that regenerates and has a cap, even health in games with health regen.)

Anyway, my question is, how do you want the rate of mana regen to change with your current mana? There are three cases that I have seen:

1. Mana regenerates faster when it is low. (Ultima 7, where you recover half your used MP every hour.)
2. Mana regenerates at the same rate (Oblivion; while changing your max Magicka makes it regen faster, changing your current Magicka doesn't affect the rate. This is probably the most common case; Jedi Outcast/Academy are other examples, and Shovel Knight: Plague of Shadows is (IIRC) another.)
3. Mana regenerates slower when it is low. (This is seen in Cookie Clicker with its new Grimoire minigame, which allows you to cast spells. A limited version is seen in some Zelda games when riding a horse; your horse''s stamina will take a while to recover if it runs completely out, but only in that particular case.)

So, which of these cases do you like the most? I personally prefer 1, as that encourages the player to actually use all that mana.

(Remember, this discussion applies to any resource that regenerates and has a maximum, and can apply to any genre; just look at the variety of genres I used in my examples!)

Let's not forgot the hardcore mana regeneration option: regen only after sleeping.

That's actually orthogonal to what I am asking. Even if regen only requires sleeping, one can still ask the question about whether it should regen faster if it's low or if it's high.

Also, the convention of sleeping is not conventional in every genre. For instance, how do you handle it in games like RTS games or idle clickers? (Remember, I used Cookie Clicker as an example.)

thanks very useful

In RTS games, if a unit can use a special ability that requires "mana" or "power", they usually have a "power bar", similar to the HP bar, which depletes after the ability is used and starts slowly regenerating at a constant rate after that. In real game time, this usually takes less than 1 minute.

Load up on "Warmth".

The other characters I don't worry about it.

You've missed a couple of options;

1) Mana completely regenerates at the start of every battle (meaning there is no reason not to use spells or special skills in battle, and you usually jump straight in with your most powerful every battle)

2) Mana only recharges during battle (and usually starts at zero at the start of each battle), this means that you have to wait to start using your spells and wait to use your more powerful ones (and also, like above, there is no point in not using them as your mana won't carry over to other battles)

I prefer either constant regeneration, regeneration on sleep or recovery only, or option 2 above.

I prefer either no regen (mana refill by potion or sleeping) or constant rate regen. No particular reason, but I think it's easy to estimate so I can plan my mana usage

It should only regenerate if your characters rest or if they drink a mana regeneration potion.

I prefer if it regenerates the same rate.
This rate should neither be too fast (op mages), or too slow (useless mages).

Mana regenerates slowly over time (same rate), but you have the option of entering a "meditation mode" where your mana regenerates faster but you also get all sort of penalties. I am totally against the forced use of resting/sleeping for mana regen or spell recharging and have had numerous issues with the D&D system because of it.

Mana should never "regenerate" unless the player character either sleeps in a bed, and/or drinks a blue potion. Anything other than that indicates a game with bad, dumbed-down mechanics.

Not even for Wizards and the like?

I can understand your point with warriors and such but it seems that having "magical powers" as a natural part of your person would lend itself to being able to regenerate the "juice" (forgive my stupid terms) it takes to utilize those powers.

Tales of Maj'Eyal has the Time Wardens and Paradox Mages that use paradox as a resource. You can use an ability (Spacetime Tuning) to set your paradox level. It also increases each time you use a chronomancer based ability. The higher it is the stronger your abilities - it also has a higher chance of creating an anomaly. An anomaly could be anything from an evil twin stepping out of an alternate timeline to shunting yourself to an alternate timeline where everything seems to currently be on fire.

I also like the Tinker meta-classes. Their resource is steam which is produced by a set amount by craftable steam generators. Their abilities tend towards persistent effects that require an amount of steam per turn. As long as you keep expenditure below your regeneration threshold you could keep effects up indefinitely. There are times when you need a boost in power and can activate a whole bunch of effects giving you a short term boost of incredible power - better make it count.