We don't know, but it's logical to think so. If a lot of users download ML simultaneously, chances are Apple's server bandwidth will be attacked like crazy.
Over a fast FIOS connection, given that you don't jump when everybody else does and cane the servers, about 15-20 minutes...Over a slower connection, much longer...As posted, without knowing your speed it's impossible to say..My figures are based on my own connection speeds.
Doing the upgrade when the whole world of Apple is trying to do it too will obviously take longer...
It was a pain to download Lion for the first few hours last year. Same with major iOS releases. So I went to bed and woke up in the morning and it was up to speed after