TLDR - 3-5 times a week for 20-30mins to start with and build up from there.

This can be a classic ‘it depends’ question...  If you simply want to maintain your fitness, or you are starting your journey 2-3 times a week for 20mins could be enough.  However if you are more advanced or have more specific goals your needs will be different. 

Where do I start though? You would start with 2-3 20-45min sessions. You'd walk, maybe jog, lift some weights, and you'd probably see results in a few weeks (improved sleep, energy, stamina, etc) maybe even find that clothes fit better.  The main objective here is getting your total activity up.  And remember something is better than nothing. If walking for 30 mins is not sustainable, do 30s of walking, 30s of standing for 20mins. Do laps of the lounge/garden. Just start.

Your primary objective is to start a habit of fitness. What ever you choose to needs to be achievable time and effort wise, and arguably more important, sustainable. If you are unable to sustain your habit of fitness, you will lose interest/time/focus/will power. Trust me, I've started and dropped a million programs due to my own unrealistic expectations. Keep it simple and repeatable. 

Once you have built a routine of fitness you can start to increase what you are doing, things like load, intensity and frequency.  The key thing here is that you need to build gradually, not suddenly jump into 5 a week heavy intense sessions.  You can do this, but your chances of not being able to sustain it or worse get injured increase.  Slow and steady wins the race for you. 

Regardless of where you start, increasing your activity levels, no matter how you do it, is going to bring you some benefits. Walking in a forest 3 times a week won't make you look like Arnie, but it will improve your mental health feels (technical term there).  And pumping iron wont' give you the mobiliyty of an acrobat but it would increase your confidence in helping a friend move a couch, or get all the shopping from the car in one go. 

So find 3 20min slots in your week and start