Canal cruises in February in Bruges and Ghent? Approximately how long?
They aren't "cruises", they are small open boats which have 15 to 20 tourists and somebody driving, with either taped (in many languages) or live commentary (always live, I think in Gent) which go around the same route of the canals regardless where you board - around 30 to 45 minutes. It has been a few years for me, my timings are by memory, the boats were less crowded in Gent, the route was more beautiful in Bruges.
The Bruges landing stages have always been crowded with boats leaving every few minutes as soon as they are full. I haven't been on the boats since the Chinese/Indian tourist invasion but I expect that they are that bit more crowded.
When I was in Bruges last year I rented a bike and rode to Damme along the canal. As I rode I passed (or was passed by) a paddle-wheel river boat. Here's some info about it: https://www.visitbruges.be/en/lamme-goedzak-steam-wheeler-damme-2.
These cruises and their canals are less substantial than in, say, Amsterdam or Strasbourg. Don't expect too much.