There's no limit. There's a 150k or 300k ESTIMATE, which means that your camera will definitely reach that mark without problems, or at least somewhere around that figure. Also, for amateur users, typical usage will not be more than 20-30k per year, but for someone like me who is pursuing photojournalism, i can easily hit 100k a year from now on. (I hit 40-50k last year in 2009 because I didn't touch my camera for almost 4 or 5 months). It also depends how economically you shoot. I don't shoot 20 frames for each composition. I shoot economically. If you're using a high fps camera and you're a typical amateur you probably will use the 8 or 10fps even if you aren't shooting sports, and that's going to increase your shutter count real fast.
Anyhow, it isn't expensive to replace the shutter. Maybe $300.