If you're not paying up front, typical rates are between 30% and 40%. 1/3 is typical.
If you pay up front, it's much cheaper. Hourly rates vary from $150 an hour or so up to $700 an hour for top partners at major NYC firms. That's typically cheaper than paying a percentage of your settlement, but you're on the hook even if the case goes to trial, you loose, and you get nothing.
Which scenario is typical depends on the type of case and how much you have to pay depends on where you are, the nature of the case, and the level of experience of the attorney.
You should have all this information up front though. It's in the retainer agreement. If you are hiring a lawyer, you'll be given a retainer agreement (typically 1-2 pages and not too dense legalese) to read and sign before anything happens. If one is being provided to you from your union or some other arrangement, ask the people who have arranged for the attorney or, better yet, ask the attorney directly how they are paid and how much.
EDIT: Reading your question again, it sounds like you might be talking about a class action. If so, it varies even more widely. Typically 40% is common in a class action because the individual members of the class have little control. Theoretically the other side is supposed to pay for the legal fees in a class action, but that's not a huge factor in a settlement, it comes more into play if you win at trial. Depending on the lawyer and the judge (the judge has to sign off on class action settlements as a check to make sure they are fair), it may be very fair to the individuals, but often times the individuals get screwed over. You will have the option to opt out of the settlement and get your own lawyer. This option is difficult and expensive and may or may not be right for you. If you want to know if that's what you should do, contact a lawyer to see whether or not they think you should pursue your own case. I can't tell you that on Yahoo Answers because I cannot give you legal advice.