Are we talking about buying or selling?
As a listing agent, I've had listings that have taken hours to sell, and I've had listings that have taken over a year to sell. As a buyer's agent, I've had buyers that I've worked with for years.
As long as you feel the agent is doing what s/he should be doing... providing you with the necessary properties to look at, sending you e-mails, communicating regularly, doing comps & CMAs for properties, or if listing; marketing your property well using brochures, print ads, internet advertising w/expanded photos, virtual tours, enhanced descriptions...
As long as you feel these things are being done properly, continue to work with that agent. That could be a month, or it could be years.
It's also possible you could determine within a few short weeks, that the agent is not doing what s/he promised... that s/he is not marketing, not returning phone calls, not sending e-mails or communicating in any way, and is a bad fit. In that case, you may want to terminate the relationship early.
It depends, entirely, on the agent in question.