Now be warned that most sites will require you to at least click 4 or more ads a day to earn from your referrals so not hitting those click you are going to lose out on your referral earnings. So make sure that if you sign up for an amount if sites that you will be able to click at least the minimum amount of clicks each day.
So if you decide on 10 different sites, figure at least an hours worth of work. Now each click might only be 10 or 30 seconds but my time you sign in and check how your pay and referrals are doing. I would say your going to spend at least an hour in front of the computer getting all your clicks in. If you can not do this amount of time each day than you might not want to sign up to ten sites or at least not get referrals for all these sites.
At first you might take longer as you try to figure out the sites and how best to spend your time. Once you get to know them this time will be cut down.
Make sure not to spread yourself to thin and lose out on income, make sure that you are making the most from all the sites you are working with. Than if you see that you have additional time start to take on more and more sites till your at your maximum level of free time. The more that you can work the more earnings you will make.