Do lands tapped for mana enter the mana pool immediately or wait for spells to resolve?

When you activate a land's mana ability by tapping it, the mana is added to your mana pool immediately as part of that ability's resolution. Mana abilities are a special category: they don't use the stack and resolve instantly (CR 605.3a). There is no waiting period.

Your mana pool holds the mana until you spend it or until a phase or step ends, at which point any unspent mana empties and you may lose life equal to the amount lost (CR 500.4, CR 119.3). This 'mana burn' rule was removed in Magic 2010, so today unspent mana simply disappears with no penalty.

Because mana abilities don't go on the stack, opponents cannot respond to the tapping itself with instants or abilities to prevent the mana from being generated (CR 605.3b). The mana simply exists in your pool the moment the activation is complete.

Concrete example: You tap a Forest to add one green mana. That green mana is in your pool instantly. You can immediately tap another land, then spend both mana to cast Grizzly Bears — no spell needs to resolve in between.

This applies equally to non-land mana sources like Sol Ring or Dark Ritual — but those are instants/abilities that do use the stack, so the mana arrives only when they resolve. The shortcut for basic lands is that their mana abilities bypass the stack entirely (CR 605.1).