I don't believe in setting "goals" per se...
Not as much as I believe just getting into as good as shape as you possibly can, as healthily as you can, and as patiently as you can.
That being said, many sayings about "age" abound.
For example "you are only as old as you feel"
If you are healthy (by a physician's standards).. then I would start off easy with some walking, swimming, biking and moderate weight lifting.
You cannot possibly know how many days a week to hit the gym... as if there is a "set" number.
How is your nutrition? Do you know much about nutrition?
For example, do you know how many calories your body burns at rest vs. how many it burns at work (give or take, it's not an exact science)
Your caloric intake will have EVERYTHING to do with "how many" days you are in the gym.
You should not be in the gym when you are starving, nor should you be in there when you are.... full?
How many days a week you are in the gym should be based on your individual progress....if any. Are you making gains? Do you know how to assess gains?
You can't just stand on a scale. Body fat levels need to be tested. Ideally, in a perfect world, losing fat and gaining muscle is key,especially since fat ONLY burn in MUSCLE TISSUE...
So...get familiar with a body fat scale or calipers.... and.. the mirror is also a great guide,as long as you are using the same mirror with the same lighting.
You would not believe how much difference a type of light can make on a face... or a physique... and there are SO MANY types of lights these days, and most of the lighting in gyms is geared toward making you look BETTER than what you really do. ... for obvious reasons.
If you are making progress on one day a week, why would you go more? This is just an example, but so many people have this attitude that "if a little is good, more must be better" .. and this just is not true when it comes to body building, sculpting, fitness, whatever name you want to put on it.
Test yourself on pushups. How many can you do today?
Do them, count them, don't cheat. Take three days off and do the same thing again. All things considered, if you did more the second time around, you are kind of uncovering the answer to your question.
I know that I have an elusive and odd way of speaking, but if you can get past that, you will find some value in what I have to say... and I only say this because I have been helping people of all types, sizes, genders, ages and disabilities... get into top shape for over 30 years now.
Test yourself to find the answer.