Yeah, how much time you get to dwell on it will depend on lots of factors and the needs of your client/employer.

I happen to work at a place where timeframes for fixes are quite lax, and the codebase I'm working with is slowly being recussitated from a decade of bad decisions from a D-tier web developer. (Dreamweaver was involved... let's leave it at that.) I can imagine if you have a codebase that was somewhat competently designed from the start and/or your client/employer has a "I need this fixed yesterday or else your ass is FIRED" urgency, you don't have time to dwell on such things. But it helps to have the experience that at least lets you do it a little bit rather than not at all.