i am currently re-installing the OS ---how olg does this normally take.....longer than 3hours???

Usually an hour or less.

If you've downloaded and installed the OS package on your computer and installing from then should take no more than an hour, as Dubdub says, and certainly no more than an hour and a half.

If, however, you are letting Desktop Manager do the upgrade automatically online, some have reported an extremely long time. If that is what you are doing, then you need to leave it until it is done.

Really, either way you need to leave it alone until you get either an error or a completion message, or until it is done.