It's a safe bet that most people are out of business for less than an hour. Of course, you'll probably be spending hours exploring all the new features, so you might want to count that against your system being unusable for "real work" due to "exploration and play".
Since you can't easily interrupt things and won't have use of the mac once the installer logs you out to start the upgrade, do give yourself a little window in case things run long.
Here are some ballpark numbers people are reporting for upgrades with real life amounts of data on their macs:
- iMac / Mac Pro + SSD: 8 to 15 minutes
- iMac / Mac Pro + HD: 12 to 20 minutes
- Portables: 15 to 30 minutes
- full drives / Air with HD: 30 to 45 minutes
Furthermore, you won't really know how long it will take if problems crop up moving thousands of tiny files or the process hangs. Then you will need to research what to do if trying again doesn't sort things out for you.
Even if your install should take 15 minutes, why risk it until the pressure is off?
The problem with any prediction is that there are four parts to the install:
- common prep tasks (pre-download any updates - lion won't have many to pre download now since it's new)
- a file system check of all your data (to avoid problems with bad files or bad file system structure)
- the standard install (move the old, write the new - most macs capable of Lion will be within 50% of each other for this part)
- upgrade script to crawl through all the your files and programs, upgrade things, and then delete the now un-needed files.
Parts 2 and 4 are where "slow installs" take the majority of your time.
Most people will be done in less than an hour - but you can't really know if your install will go long until you let it start. It matters little that other people had a good experience if your is going to be especially slow.
You can run a full file system check before starting to be sure your directories won't trap the installer in an infinite loop, but other than deleting things like un-needed apps and files that might need "migration", you can't speed up the parts of the process that depend on your pre-existing data.