Ruskie, Ancestry say about 6-8 weeks, but it can be longer. One of mine, sent just before Christmas a couple of years ago, took around 11 weeks to show as received, but things then seemed to move more quickly than the estimates for processing times.
Kits posted in the UK go to a collection centre in Ireland, where they can sit a while until they have enough to send in a batch to the lab in the US. They are only marked as received once they arrive at the lab and have been scanned in I believe.
There is a DNA Processing Times message board on the Ancestry website. When I sent mine, there were regular threads running where submitters were updating with dates sent, received, processing and completed. Just had a quick look and there don't seem to have been any similar threads recently, but you could ask on there if anyone else has had a receipt for a kit submitted around the same time as yours.
https://www.ancestry.co.uk/boards/topics.dnaresearch.dnaprocessingtimesOnce those threads start, people will often contribute. It's just a matter of getting the ball rolling.
Just a thought, but I wonder if the stoppage on sending foreign mail following the Royal Mail cyber attack would have affected delivery to Ireland? Depends when you posted it of course. I think that started early to mid January and has only recently started moving again.