If is the exact mesurments as the stock its 20mins to an hour. Some people are quicker than that. You take out the four engine mount screws. Disasemble all linkages, then unasemble your cb, clutch,flywheel, and put it on the new engine. Not hard at all.
I recently did the exchange with HPI . I sent in the .21 with my 99.00 + 15.00 for the 3 shoe clutch since they made it sound like id have it in a few weeks. It turns out both the engine and the clutch were on backorder. I sent the motor back in october and got my new .25 in mid December. and I just got my 3 shoe clutch last month. Needless to say I was pissed about missing some of the best snow bashing of the winter. but It turned out to be well worth the wait afterall. Just call them first and make sure they have parts in stock so you arent waiting forever with no engine