It probably takes an hour or less to cut through the sealer to get the windshield out. It's easier with two people if you use the piano wire tools to cut it. Even when the gasket is completely cut, from my experience the windshield won't just push out. I always had to slowly work it out and usually cracked it anyway. It's easy to crack them for sure, but if you aren't worried about cracking it then it will come out quickly with force once the sealer is cut.
The hard part is removing the old sealer. It's been a decade ago now, but I remember I wire brushed it out with power tools and it took me at least 3 or 4 hours (maybe it just seemed longer than it was, it's a thankless job). Like I mentioned before, the "pros" that I dealt with sure didn't bother removing all the old sealer and just installed the new windshield with new sealant right over the old. Two times I paid like 250 bucks and both of them leaked, which is why I eventually did it myself. It's probably not necessary to remove all the old sealer under ideal circumstances; i.e. your windshield wasn't leaking, the old sealer was bonded well and you're careful how you cut the old sealer.
Also wire brushing the sealer out removes some of the paint, which will need to be touched up. It's all covered by the windshield trim so it won't be exposed. Read the label on the sealer first to make sure the paint you apply is compatible.