It depends what kind of caster and martial you are talking about where the balance occurs. If you are talking about Wizards, most of the martial classes are more powerful than Wizards in tier 1 before they get subclasses (generally 3rd level) and balanced for a while after that.
After subclasses are in play, if we are comparing the better Wizards to the better martials:
Monks are balanced with Wizards level 3-4, behind levels 5-13 then balanced again levels 14-20
Fighters are better than Wizards at level 3-4 and balanced from around levels 5-9
Barbarians are better than Wizards at level 3-4 and balanced at level 5.
Paladins are better than Wizards from level 3-11 and balanced levels 12-14
Rangers are better than Wizards from levels 3-12 and balanced levels 13-16 (note a Ranger optimized like this is not really played like a martial)
Note I am assuming the best subclasses from those martials and optimal builds on identical average rolls.
Cleric is the only caster that I would say starts ahead of some martials and never falls behind them the whole game.