all the way in close to the base of the needle. I use a 25G 1 in for all my injections. I do delts, outer quads, and glutes. I have low bodyfat so I get away with the 1 in. for the glute but I make sure it is in all the way. 1.5 in is better for glute though. As long as you have a big slab of muscle at the site you will be ok. If you are really skinny then you might want to use a smaller length.