40 - 50 seconds for the first ones, up to 30 seconds for the mid show one currently, no doubt they'll increase it down the line.
When I was messing with it yesterday was getting 2x 28 second unskippable ones at the start of anything, a 30 second one mid-show and often trying to skip backwards or forward would trigger a 28 second ad. Then randomly any mention of ads disappeared and just checking now there is no ads or any mention of it in Prime Video itself, but there is the entry in account options to upgrade to ad-free. Dunno if they are having technical problems.