Simon Denham, founder of Capital Spreads, in an interview earlier this year which you can find on youtube, estimated the average bet with CS to be no more than 5 minutes.
Not saying that's a good or a recommended thing by the way, as it depends entirely on your style and how the market is running.
I'm not sure that aiming for a particular length of bet is really what you want to be concentrating on. You have to know what your target(s) is(are), and you probably should be clear from the outset whether you will close it before the end of the day, or if you are prepared to let it run overnight. And you also have to be clear when you are going to call it a failed trade and exit (i.e. have a stop loss and stick to it). Other than that, the market will decide how long it takes.