In answer to your question I would say, you should practice as many hours, or minutes as you enjoy practicing. It isn't a sprint, it is an avocation....Patient, persistant, practice... again and again, and.....

I would suggest that you read the recient forum thread titled "Playing for the fun of it". It has very good advice on how to appraoch the long term learning of any skill like musicianship.

I would also suggest you start writting songs as soon as you can manage to put a few chords together. Dont worry if your early work sucks. Bob Segar once answered Glen Fry's question about learning song writting... "What if I write a song and it sucks?" Segar answered...."Oh yeah... they are going to suck, but don't give up. If you continue to work on it, you will eventually write a good one. That is how you become a song writter"

In answer to your question I would say, you should practice as many hours, or minutes as you enjoy practicing. It isn't a sprint, it is an avocation....Patient, persistant, practice... again and again, and..... To paraphrase Lisa' sage words, "You will be able to do all you want to just as soon as your body is ready".