Question: asking about the lemon propagation timing in field and as well as in nursery
Answer: suggested to apply or brush on rootex  or rootone hormone for speedy growth of roots  in selected parts of plant along with peat moss mud 
timing=   may to august,  the  air -layer is then left alone  for  3-7 weeks to allow roots to allow roots to form. the air layer should be checked after three weeks for root formation.