I’ll echo what Ali_B wrote above. Only you can figure out how much delay is bothersome to you.
I know that for me the answer varies widely by what instrument I’m playing. When playing drums I start to feel it and it’s unpleasant somewhere between 10 and 20 ms. I can go materially higher when playing guitar or keyboards.
In any case, the delays in your sound chain are additive. I had a goal to keep mine under 10ms in total just because I felt like it.
I use a Line 6 G10 (3ms) → RME UCXII (~6ms) → Shure PSM300 (<1ms) and that gets me where I wanted to be. A cheaper IEM I was considering at the time was around 6 ms and that would put my latency high enough I paid a little more for the PSM 300 (which is analog).
Sound travels in air about 1 foot per millisecond. (Ali_B said 3ms per metre, but I don’t speak British.)
With decent modern equipment, a wireless setup including IEMs is probably lower latency than the old standard “cables and air” sound chain on a typical stage.