What I do is fill it enough to where you put your finger in it and it comes to the first line on your index finger. Heres another idea next time you change the cage put a small paper bag for the first layer then put 6 count them 6 layers of news paper then put the fluff in it it will make changing the cage EASIER
2-3 inches. You can also put more in for the winter. Some guinea pigs burrow in it like hamsters; some don't. Proud owner of 7 guinea pigs.
Just enough to cover the bottom of the cage. Guinea pigs don't burrow, so if you use too much, you're just wasting money.
Just enough to cover the bottom and with my guinea pig we put newspaper under the bedding because it is a lot easier when it comes to changing the bedding.