Well I told you that you were asking me to dig deep. Have a look at the reference manual for Modelsim:
http://www.actel.com/documents/modelsim_cmd_ref_ug.pdf The run command syntax can be found on page 170.
Now, I had to wipe my machine last week and re-install everything and I haven't reloaded ModelSim just yet. Otherwise I think I could answer your question precisely. But up in the toolbar is a little number spinner box that specifies the run length. I believe when you type run, it runs for the length of time specified in that box. It sounds like you can control this value from the modelsim.ini file.
Is there any particular reason why you wouldn't just specify how long you want the simulation to run when you issue the run command?
Jake