I read somewhere it says 40+ and somewhere else 60+ and somewhere else 2500 RPM+ so I'm not entirely sure lol. I just follow a lorry.
To clean a dpf you have to have the engine fully up to temperature. Running hot enough so it can create enough temperature to burn off the crap stuck to the filter essentially and keep it running at a decent enough speed to burn everything off. So not stuck in traffic or at lights. Essentially motorway driving is best.
This means that speed, revs etc don't matter as much as length of journey to get it up to temperature. It's fairly obvious if you are going on a long journey it will at points have moments where you are traveling at speed with constant revs enough to help burn the dpf.
My recommendation would be at least one 45-60 minute drive on a weekly basis. I've had a few diesels in the past and the one with dpf would start to regen just as we were arriving home from a 30 minute drive. It wasn't driven long journeys regularly. Luckily never had dpf issues but I suspect an older car would be more susceptible as it clogs more with all the crap spewing out of it.
You need to do a long drive regularly. Forget speed or revs. Drive normally for a long time which means constant speed on a motorway is ideal.
I imagine 40mph or 60mph makes no real difference other than 60mph gets it done faster but at that speed you have driven a longer distance in a shorter timeframe anyway 40mph would work just as well if travelling the same distance overall as it takes longer to get to the destination.
Driving at 3k plus revs is just another cheat method of getting it hot and keeping it at high temp for a quicker regen on a short journey. It's not needed if you do a long journey regularly. It also will likely damage other components quicker running it at high revs especially an old diesel causing the turbo to blow possibly or prematurely.
If you have to baby the car because you don't do long journeys it's the wrong car should have bought a petrol. Forget speed or revs and just do long journeys. If you don't do long journeys then get a petrol.