How long should you practise? Well, there's several answers to this, to my mind:

Practising a little every day is essential - on your own, to struggle with the new stuff, is helpful for embedding it. Especially if you are getting it wrong on your own, since that means you are learning how to learn. Never worry about getting stuff wrong, it's  the practise itself that's important - everything will come right in the end if you keep on trying.

Practising in a group is also essential, whether you do that every day, or just weekly. Having other people to practise with, means you can all help and encourage each other, as well as having a social group with a shared interest, which is nice. Also, if you just practise on your own, you can't  progress onward and upward. Not that there's any time limit to reach any particular goal, but it's nice to know that your practise with others is helping you make that progress.

Perserverence is on my mind, because while I am progressing, there is one aspect of tai chi that I find very difficult to master and that is Pushing Hands. I was made aware of this by my own reluctance to attend an intensive on PH training. After thinking about this reluctance of mine, I felt it was due to my not wanting to fail yet again, so I decided this was the wrong approach and signed up for the intensive. I thought that fear of failure was more to do with ego than anything else, so I decided not to worry about it and just get stuck in.

By the end of three hours, I  felt happy that I had had a go - it's obvious I need *plenty* more practise, but having a go meant I got over my fear of failure and just enjoyed doing what we were all doing, which is the best way to practise anything. So now I shall watch out for those little reluctances as they crop up and regard them as steps to progress. I remember reading in one book by a Japanese teacher that every time you feel you are about to fail horribly, you should regard it as the prelude to stepping up to the next level, which is an encouraging thought.

Anyway, keep practising and perservering, let go of those ego issues and enjoy the moment with good friends! Be happy in what you are doing and don't worry about being great or terrible at anything, just do it!  :)