I rode with Freeride Madeira earlier this year and thoroughly rate them, good bikes, good group level, great guide and amazing trails (lots of variation).
If it’s just time you’re worried about, the day finishes relatively early (think about 4.30).
I’d agree with all the other posts, anything less than a day isn’t really worth it.
If you really can only do half a day, the locals use the bus service from Funchal up the mountain (couple of euro each time). But you will still have to find the trails down on your own (not impossible, but it’s not a bike park so won’t be too easy). You’ll probably only get two runs in half a day.