Hard to say.  I know guys who cannot fly very comfortably in any wind (or even when it is calm) and others who are fearless and very skilled that fly very well with .40 size models in 20 mph winds.  Depends on model selection (symmetrical wing & heavy loading), skill level and whether the wind is steady or gusting and changable in heading.  As a rule of thumb: faster models are less effected by wind.