There was a thread on this recently - worth searching for?
I am a PG, have done nearly one year and thoroughly enjoyed it - but I suppose it makes a difference that I like the school, the Head, and the other Governors (more or less).
It can take as much time as you want it to, but a minimum of the Full Body Governors' meetings (we have 2 per term, they go on for 1.5 hours usually) and one National Training day. I've done some extra training, and am involved with two committees (Personnel and Policy/Curriculum) so have done quite a bit - especially as we had a Head to appoint, are going through a lot of change, etc.
I used to be involved more in the PTA - helped set it up, and was Vice Chair - and still am, but less so as less time/energy. It does feel like a different role, but both are equally helpful I think...maybe check out the PTA, and ask if you can sit in on part of a Govs meeting?? Our school offered the latter, when I was dithering - made it less scary.
Depends what you want to achieve from it. If you want to be hands on, then PTA is the way to go. It really makes a difference to the children and has great satisfaction when you purchase x, y and z.
Governor is more paperwork and policies and you may vote or hear things that will affect your child/children and you may not like so our Head warns anybody re being a parent governor that they have to set aside their personal feelings and do the best for the school.
Being part of the Governing Body can be a really interesting thing. Beware, however, that it can sometimes be a big deal. A friend of mine had the summer from hell last year because there was a disciplinary matter that involved the police, the governing body, bigwigs from the teaching coucil and a member of the SMT at school. It was stressful and required hours and hours of her time in meetings. That is a very rare occurance however.