DS1 about 18 months, but he is 13 now and it was considered weird/extreme back then. These days I'd definitely RF longer than 18 months. I moved him into an impact shield seat because at the time it was shown that they had lower neck loading, which is what I was worried about with FF. However newer evidence has come out about them and it turns out they are no more protective overall than a normal forward facing seat because they have higher instance of ejection and of abdominal injury, which isn't detected by crash test dummies. Ejection because if you crash the seat directly head on they don't come out (and this is how lab tests are typically done) but they do when you do an offset crash test (where the corner of one "car" hits the corner of the other - much more realistic in terms of how head on collisions go). And abdominal because until recently there were no crash test dummies with abdominal sensors. Even now, they are separate dummies so most crash tests aren't performed using them.
DS2 was just over 2, but I wish I'd pushed it a bit longer. We have a Joie 360 Spin for him and I started a new job which finished at about his bedtime and DH would come to pick us up. He started to get frustrated about being in the car seat and me sitting in the back didn't always help so I suggested putting him FF occasionally if it was really bad. I kind of meant it as something to do as an occasional one off, but DH preferred it massively and would drive him around like that all the time. When I was in the car it made me really anxious. Possibly a failure of communication, rather than anything malicious on DH's part. We kept him RF for long/high speed journeys but after a while there was always some reason/excuse even for those and I gave up and got a better rated FF seat when he was about 2.5. He was also 16kg then so getting close-ish to the limit of the Joie anyway.
DH drives, I don't, I don't police him enough to dictate how he has the car seat when he's alone with DS. He did say he would have turned him back RF if I really insisted/felt very strongly about it but by the time we had that conversation I felt like it was too late because DS2 absolutely loves FF. He is now nearly 3 and I feel completely happy with him FF. I chose the seat carefully (Britax, isofix, top tether) and ultimately he is older and less vulnerable now. I know he would be safer rear facing, but I don't personally think life is always about doing the safest possible thing, it's about doing something that you're happy with the risk level, which works for your family and situation. Other people really do prioritise safety and probably think my reasoning is flimsy - that's OK - we all have different priorities.
DS3 is due around the same time of year and I think I will insist on RF until at least 2.5 to get him through that third winter, and then maybe look at his weight - if he's close to 18kg (predicted to be bigger than DS2) then we might switch him to DS2's current seat, which is harnessed to 21kg, or keep DS2 where he is and invest in a 25kg RF seat. Or I might be able to drive by then in which case he will be RF in my car.
We live in Germany now and I think ERF is much more common here, I often see toddlers rear facing out and about, I think there are 3 ERFs at our nursery, which isn't huge, whereas people in the UK seem to say they don't know anyone else who does ERF! It's also so much more accessible these days everywhere, with the spin seats being sold in high street stores and the modular seats like Maxi Cosi etc having a rear facing option now. Much better than when DS1 was that age when it was just impossible. I probably could have realistically kept him RF to a maximum of 2 using the Britax First Class Plus.