I've seen some higher % commissions, lots at 15%, and a few lower %s. But you should have your contract reviewed to see if it's fair otherwise, and you shouldn't assume that a contract that offers 15% is fair or that that's the only provision you should be concerned with. For example: is the 15% is of your gross, adjusted gross or net; are there minimums that have to be made before you pay commissions; how many years does the contract lasts; are there sunset clauses, and if so, what do they say; is anything is excluded from the commissions; how can you terminate' what's the scope of services; how are loans treated, etc., etc., etc.
How well do you know this manager? Do you know other clients the manager has managed?
This is an important relationship, and you'd be less than truly serious about your career if you signed a management contract without having a lawyer review it.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
I somewhat agree with the previous posting, but more frequently see 20% than 15% (which may be more typical for a newer manager). I just want to add that you want the scope of the management agreement to match your manager's experience. For example, if you hire a comedy manager but that person has no experience in other areas of entertainment, then you don't want him to commission from your television career when he had nothing to do with it. You may end up with multiple managers so be careful as to who commissions in what areas.
I have seen commissions range from 5% to 20% of the gross received by the client. Generally speaking, with my clients, I usually try to keep managers' commission in the 5%-10% range as they are there to assist the client in the management of their career, not act as an agent. This being the case, a client's agent will be doing a majority of the work for their 10% and there is not reason for the manager to take more than an agent if their service do not generate fees for the client as often as the agent does.
As the other two answers state, you do have to be aware of other terms in the agreement as well as the experience of the manager and their areas of expertise. I would not have a writer sign with a talent manager. You should hire an attorney to review the agreement at the least - it is best to spend the money upfront to ensure that the manager and his/her contract will provide what you need instead of spending a lot more money later if the relationship goes bad.