That depends. But the pattern is simple enough: I reach a point at which my damage output is too low. Generally, physical damage. Characters with healing spells just don't cut it in melee - and casters eventually run out of mana, and then they die. And I have ranged skills trained on everyone obviously.

I've died at the last two levels of PC, died to Keethrax, died quite many times in SMC. Started CoC only twice, and reached Dwarf Town only on my troll beastfighter.
Look at it another way - if you reach a point where your damage output is too low, then you've been pushing ahead too fast, and should have been doing something else, somewhere else. There are a number of different places where low level characters of all colors can thrive. The infinite dungeon, for example, is still a great place to get some experience and spellbooks even without abusing the stairs. You'll find damage spellbooks sooner or later.

The fight against Keethrax is frequently delayed even by experienced players because it's quite simply a tough fight, and there's no time limit on it.

All that being said, just using the best weapons I come across tends to be enough for me whenever I play spellcasters. I worry about defenses, particularly PV, far more than I worry about damage.