Doesn't look like it should take too long, I'd get it out of the box and look at the instructions as there might not be too many parts to fit together.

We are going to gradually piece together our trampoline over this week and then on Xmas eve DH is going to get it out of the shed and put it in a place in the garden so DD can't see it and then when she is in bed we will put all the bits together. You would have the same problem that we have - good or bad - that it doesn't get completely dark until after 9.30pm at night so can't sneek out after dinner and do the assembly. I'm just hoping we can use the good old 'bed early as santa won't come until your asleep trick', I would hope that would work for Master B as well .