I dry hop for about 7 days before I bottle/keg the batch. Depending on how much you want the hops to be noticed, will determine how much to use. But, I wouldn't go over about 3oz in a 5-6 gallon batch.
I actually kegged an English IPA that I dry hopped with 2oz (1oz each EKG and Fuggles) last Wednesday. One keg is in the brew fridge, on gas, the other is in reserve. I'm looking forward to drinking this one.
Depending on if you're bottling, or kegging, you might want to use something to help keep the large hop particles in the fermenter. Although, I did have a few pints of the last batch that had some small hop floaters. I'm transferring via CO2 pressure these days. For batches that are dry hopped, I've used the small cap that goes onto the end of the stainless racking cane. It got a bit plugged up with Wednesday's batch, so I might need to use something else in the future. Although, once I had the QD off the keg end (I like filling via the liquid post) the transfer went well (there were particles in the QD).
Depending on how much you like Amarillo hops, use that to determine how much you use. Maybe start with 2oz for 7 days before it goes to bottle/keg and see how you like it. You can always add more, or less, to the next batch. Until you do that, though, it will be hard to know how much YOU want to use in the brew.