For the classic NES games and Super Castlevania IV... the time limit is kinda... just there. You really have to go out your way to actually run it out. I imagine it's main purpose is for bonus points at the end of each stage. The Metroidvanias though, which are Symphony of the Night and beyond, have no timer whatsoever... so you're free to go about the games in your own pace... well moreso than you already could.
The only real Castlevania game with a hard time limit is the N64 ones where you need to beat the game under a certain time to fight against the true last boss... but even then, there is no real hurry in finishing anything.
