It would depend on like ldy said basically, the locality. I have no idea what local petco(shivers!!) boas are but I doubt they will live past the 2' make as it is the way they are going. Just kidding of course
When you don't know the locality, its always best to PLAN for a monster just so if it happens, you won't be in a bind and shocked. I tend to plan for a 4' by 2' cage at 2 1/2-3 years of age for any non dwarf boa(I plan for a cb-70 tub to last any dwarf boas 3 years+ depending on locality).
Just keep in mind that there is no way to predict their actual size. Even a hog island which normally maxes out at 6' can hit 7'+ if fed agressively over a period of years.