There's no need to rebuild each time you make code changes, if you're using a website. If you're using a web application, it will need to be rebuilt in order to debug properly. Also note when you make changes to any piece of code in the /App_Code directory or the web.config file, the site will rebuild automatically. So if you stay out of those areas, you'll also be fine. If you're using a web application/project, things will build a lot faster but you also have to deploy the whole site when you make a simple change.
You can make a few setting changes to your website so it doesn't do this each time. Kentico MVP, Brian McKeiver has a blog post about this
Also if you have the ability to upgrade your machine, go with a SSD vs. a traditional spindle drive, you'll notice improvements hand over fist.