If you are building your modules as WAP modules, then every time you build your project you are dropping an assembly in the website bin folder.  This action will cause an app restart which means DotNetNuke must completely reload all the assemblies and reload its cache.  This time will be a fairly constant amount, all other things considered.  As far as I know this is tied directly to the speed of your machine to load all those files and make the initial db calls.  If the project you are working on is a WSP based module then no dll is created and refresh times are considerably faster since controls then are only compiled when they are actually used.