As long as you use a good quality compound you can go to a depth of 10mm. You can also use a filled mix to go deeper or even a deep section compound which will normally let you put down up to 50-60mm deep.
But it is hard even for a pro to put down compound reaching a depth of 10mm in certain areas and much less in others. You have to be very fast with loads of control. Other wise you will end up with the subfloor worse than when you started. I would always advice to go over the subfloor with a straight edge and mark out any big low spots. Then mix some compound up and fill them so that the subfloor ends up half flat and smooth. Leave for 24hrs and then level the whole floor out as one. Much easier and you will get better results.
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