You say "quite some time". If this really is quite some time, you might want to fork a process to do what needs doing and send a thank you page back to the browser that sets a cookie identifying the fork and quit.

Get the forked script to store the cookie ID in a DB/log file somewhere. When the fork finishes, it removes this ID from the DB/log file. When the user clicks to see if the processing has finished, read the cookie and check it against the list of running process (fork) IDs in the DB/log file. If it's finished, proceed, otherwise send back a still processing message.

Or something like that...

cLive ;-)