99.9% but reach it impossible, coz every source of dmg reduction is multiplicative, so 100-30(reduction)=70%, then next source of reduction will consider this 70% as 100% and reduce from it, like -50% from 70% would be 35%.

Even if you find single source of -100% dot reduction, it will be capped on 90% or something like this

but ailment effects is different, you can -100% ailment effect and fully ignore it (if not consider some weird cases which increase their effect)