It isn't too difficult to get the old vents off, if you have a heat gun or even a hairdryer you can soften up the sealant and it scraps off fairly easy. Just don't point it straight down, let the hot air blow across the sealant. I use a scraper that I rounded off the corners of, so I didn't rip the roofing material.
Once you have the sealant scraped away so you can get to the screws, remove them. Now you take the scraper and work it under the vent. When you have gone around the vent, start lifting it out, you may find some spots that need a little more coaxing to get
I never actualy timed how long it took, but I know it wasn't more than an hour because I wasn't late for lunch. Hate being late for lunch!