First I’d like to say, all the very best luck with it! It is never too late to start to play an instrument, and whatever level you achieve, whether you play solo alone at home or in company in sessions or to perform, you will get so much pleasure, joy and satisfaction from it - certainly enough to outweigh the frustration and hard work we all experience at times as we develop ( a never-ending process!).
How long does it take? Well, there are many variables and no rules! For a start, it all depends on how readily it comes to you - some people just sail away (I have a 14 year old pupil doing just that!), others have to grind at it for weeks even to get an acceptable toot out of that damn tube! If you learn real tunes from the start (chosen to help with technique at the appropriate level, of course), once you can actually play through two or three at a steady pace without “falling off”, get out there and play them! Doubtless you will sometimes embarass yourself, but face that out. Playing with others at every opportunity is one of the best ways to develop and improve, and if you can’t join in, listen (and watch)!
As for sessions, it all depends what your local ones are like. Yes, some can be a bit catty, or stuffy, or exclusivist about what music is deemed acceptable, but not all are. It seems bizarre to me, having an “approved” list of tunes or whatever, although if a session is clearly ITM only, I wouldn’t insist on playing loads of English tunes or whatever, or vice versa - you’re in the wrong place, and a session after all relies on having a substantial degree of commonality or it becomes more of a “buggins turn” play-around thing…ugggghhh! I go fairly frequently to Welsh TM sessions where the whole point is to foster the Welsh tradition, so we mostly eschew ITM or other foreign material, even though many of us play that as well. My main local session is predominantly ITM, but we also happily mix in Welsh, Breton, English, Anglo-French etc etc. There’s one regular who gets a bit humpy about that because he only wants to play ITM, but the rest of us just enjoy it. When I go occasionally to Irish sessions in Manchester, I only play Irish…
If you have a choice, pick the right session for you. If the general standard is obviously stratospheric, discretion is best - try to join in (discretely!) with anything you recognise, but don’t try to lead a set off. Not all sessions are at that level - it all depends on who the local regulars are and who turns up on the night. Most traditional musicians are very friendly and welcoming, in my experience, and supportive of people starting off and participating - unless you are blatantly insensitive and insist on repeatedly banging away incompetently. Also most sessions have participants at varying levels of competence and breadth of repertory, so will play a mix of “standards” and more eclectic tunes at varying speeds etc. On the other hand, if the local standard is dire (it does happen!), playing with a bunch of people who are incompetent and don’t realise how bad they are will NOT do you any good! You’ll soon realise if you’re in such company - you’ll be getting better and they, despite having been at it for years, will still be playing the same handful of old standards out of time/tune…
In the end, its like any other human interaction - you have to judge the company and the mood and act accordingly with regard to your own abilities and tastes.
Give it a go, learn and enjoy!