A USB miner is a very bad investment, it will not generate you the 0,6 BTC or so it costs you to buy it.
At the next difficulty in 1 days time it will generate around 0.0032 btc per day or so for 10-11 days then it will be 25-35% less for the next 10-11 days and so on for a few weeks untill the ASICs will start flood the market in sept/oct and then a USB miner will hardly generate anything.

At next difficulty change, 13 august you will need around 312 USB miners to generate 1 btc a day(but only for the first 10 days or so, after that the generation will decrease) and they will cost you around 19 656 USD(105 USD/BTC) 187,2 btc(at 0,6 btc per USBminer)

So to estimate what you can expect to get back from an USB miner will be something like

first 10 days 5,33%
day 11-20 4,10% 9,43%
day 21-30 3,16% 12,59%
day 31-40    2,43% 15,02%
day 41-50  1,87% 16,89%
after this the generation vill probably drastically decrease so it's pretty safe to asume they will never give back more than 20% of what they cost if bought today.

In it's entire lifetime a USB miner bought today will probably generate less than 0,12 btc so you need 9 or more of them to generate 1 btc over there lifetime and the cost for those 9 USBminers is around 5,4 btc today.