The time of your shots should remain the same whether you're doing a single, double, or even a quad shot. Depending on the baskets used and your machine you may find a small variation using the different basket sizes but it shouldn't be much.
Pack as much coffee as you can into the basket and when you pull the shot do it into 3 glasses with the first 30ml in one glass, the 2nd 30ml in another and the last 30ml in yet another. Then taste each glass. I think you'll find that all thoughts of getting 90ml from an 18g basket will quickly disappear.
Java "Thin white stuff, YUCK!" phile