That's how long it took to find the satellites and download the ephemeris for each one. The ephemeris is the data it needs to actually get a location fix. It's a very precise description of the satellites movements and so expires after a few hours. Once you've got one, switching off and on gives a "warm start" which takes a second or so.

The almanac is different to the ephemeris. It's a less precise description and so doesn't expire for after several months. It takes longer to download because it covers all of the satellites, rather than just the one broadcasting it, and it is broadcast more slowly. However, you don't need the almanac to get a fix; you need the ephemeris for that. The purpose of the almanac is to give the unit a good initial guess for where the satellites are, so it can find them and download the ephemeris quicker.

If the unit has been switched off for a long time, then the almanac will be out of date and it will take longer to get an initial fix. Once you have that fix, you have the ephemeris so you can switch off and on again for a "warm start". But you won't have the almanac for another 15 minutes or so. If you switch off for about 6 hours and then switch on again, it will again be slow to get the initial fix. If you leave it on for half an hour, then switch it off for 6 hours, you should get a quicker initial fix using almanac data even though the ephemeris is expired.

Getting a fix when you have an almanac already downloaded should take under a minute. If you don't have the almanac it's partly a matter of luck whether the unit looks for the satellite in the right places, and that's when it takes several minutes. However, it doesn't necessarily take 15 minutes to get the first fix because you can get a fix without the almanac, if the unit gets lucky.

I hope that makes sense.