How long's a piece of string? There are too many variables to provide a meaningful answer, most of which will be governed by you, based on your experience, attitude towards risk and methodology employed etc. As pboyles says, still having your £50k in a year's time might be a good result. A better result would be to outperform the benchmark indices - assuming they continue to rise. And that's a BIG assumption!
Ultimately, you're the person best placed to answer your own question, based on your results to date. If you've not started trading yet, then open a demo account (or a very small live 'test' account) and record all your trades to generate your own statistics.
Like timsk correctly said, there are many variables. Be sure to set a goal that makes sense, one that is consistent with your level of experience. Setting a goal that is too high relative to your experience may lead to over-trading and then inevitable disappointment.
Keep this in for forefront, when the markets turn negative, which they will at some point, don't fight the tape. When this happens, either step aside from the markets or go with the tape and trade the short side.
Don't be afraid of losing trades. If a trade is not working, exit and look for a better setup elsewhere. 60% winners and 40% losers will net a very nice return over time.
Yeah I always do stop losses a minute after I enter a trade. Was short ZOLT when the company announced news and stock rose %22 which hurt me a lot. But now am back in the green and got my entire portfolio up a few percent from when I started which is great, probably going to take some money off the table today.
FYI I have been following StockConsultant on this forum. His picks are great. GMCR I am now up almost %15-20. DRI up 5% SPR and WU which are his picks I am up a bit on too. Guy is really good. Learning a lot from him.