Or longer
It could take a log longer if you read the wrong set of instructions, or read them wrongly because, oh, say, for example, the instructions were badly written, incomplete or out of date.
Indeed: one of the moderators pointed out in a sticky (on a related topic, but I forget exactly where), that some tutorials are very bad, a few are good: information that would be a lot more helpful to newbies if we had a link to one of the good ones!
During my own searches on this topic, I did not come up with even one tutorial I could consider trustworthy. One, for example, lost my trust by mentioning the procedure only for a Rodgers G1 Dream (mine is unlocked T-mobile G1), another by proposing the ridiculous idea of using a Windows Mobile device to write data to a microSD card, another by explicitly requiring that the user perform certain steps based only on old ROM version numbers...
I hope the idea is clear: it is as easy as you say ONLY if you happen to find a good tutorial, appropriate for your phone. Otherwise, disaster lurks, you can easily brick your phone. Or if not brick it, make a mess out of it that will take much longer than 20 minutes to fix.
One thing I am surprised to see none of them mention: after flashing with the new ROM, how does the user restore Google apps and third-party apps? Surely the recovery image was not made for this.
So please: instead of just saying it it that easy, show us an example of a tutorial so well written that it really IS that easy.