I just did a 100% water change for my 5 gallon tank and it took me nearly an hour...lol but a reason for that is that I don't have a siphon/vac so I carry the 5 gallon but it's a bit heavy so I take my time scooping the water into a small bucket. Then I rinse all the rocks/glass marbles so its nice and clean ^^ and scrub the tank and the decorations too.
When I had my 2.5 gallon it took me around 25 minutes since I could carry the whole thing XD
So I'm just wondering, how long does it usually take you to do a 100% water change? (depending on your tank size of course :o)