I’m in the process of doing interviews and company A seems to like me - however company B just booked an interview for me. If company A calls me back for a job offer, what’s a reasonable time frame to give them my final answer?

Just in case, I want to keep my options open for company B depending on their offer.

A good potential employer wants you to make a decision you're happy about. Be honest about there being two opportunities in play, but DO NOT talk money. Be concerned if one of them tries to put an "exploding offer" in front of you (e.g. this offer is only good for 72 hours). That's a shitty high-pressure sales tactic that used car salesmen trade in. Don't go for it.