That's almost impossible to answer.   The odds of winning any one specific sweepstakes depends upon the total number of entries by all participants.

Read the rules. They'll usually give you the odds that the Promotor provides.

You'll increase your odds if you enter as many times as permitted by the rules. If it's a daily that runs for 30 days and you enter every day, your odds of winning are better than if you only enter once.

Remember, it only takes one entry to win, but the odds of winning increase with each entry.

Those sweeps which offer large value prizes attract the most entries. That makes them more difficult to win.  A sweep that offered a pair of dirty socks for a prize wouldn't attract many entries and the odds of winning would be in your favor if you were one of the very few that entered.

If 10,000 people enter daily for 30 days, that's 300,000 entries.  If you enter once, your odds of winning are 1 in 300,000.  If you enter ever day, your odds of winning are better -  30 in 300,000 or simplified, 1 in 10,000.  

   blueyedgirl741

curmudgeon laid it out, so all I can add is have you read:

NOT WINNING ANYTHING? Tips on WHY:
http://www.sweepstakesplus.com/smf/not-winning-anything-tips-on-why-t63762.0.html