Never. I can see where people could rationalize, "well I am lubing it up for her". But theres the problem, if she needs a dildo that just means <u>you are insufficient</u>. (Aka your dick isnt enough for her)
I have prided myself on a sexual experience that isnt greedy. Many men only strive for their own orgasm, I always hold off until I am sure my own partner has been pleasured. If you dont have amazing sexual stamina, you can always learn to finger for the g-spot, or learn proper oral.
I've put my all into it for my sexual career and I have a pretty high approval rating. Many women say that no ones even gives them oral and several have told me I'm the best they've ever had.
Any guy worth the weight of his dick in gold does this. Unfortunately, there's a surprising number of guys that don't care. :(