You can keep going for a very long time, repeatedly casting “summon monster” and sacrificing. Even without a ring of slow digestion, eating the odd summon will keep you fed. Beware only that you will occasionally summon a crowd which may involve strong monsters.
You'll need to re-read the spellbook every 20,000 turns. After 4 readings, the spellbook will fade, but you'll probably have obtained a magic marker as loot if you keep going that long.
Eventually, all the monsters that can be summoned at that level will be extinct. Most species have 120 individuals, so you can keep going for a long time. It's quite possible to die of boredom (in real life) before reaching that point in the game. Paraphrasing Jove, the DevTeam has arranged an automatic and savage punishment for infinite summonning. It's called infinite summoning.