In my case, it was a long time - - I wouldn't do it until it was legal for me to do so. Even then, I was kind of pushed into it. Many people saw the cake I did for my dd's wedding and were very persistent. I don't think this is something you can truly prepare for - - even with the best research and the most extensive business plan, the actual doing can be scary.
One thing you need to keep in mind while you're only baking for friends and family is your pricing. If you're charging a minimal amount in order to gain the experience, that minimal amount is what will be expected later. Make sure the recipient knows the actual value of the cake and that you're only charging $XX for a limited time.