A lot of this will be speculation, so my numericals might not be 100% correct; but this is what I can say.
XP Per hour at 90 Str (Youtube Video): 75-80k\hr
Assuming ~65k xp\hr at 80.
70-80 = 1,248,441 xp; @60k\hr = 20.8 hours = 1248 minutes
80-90 = 3,360,264 xp; @65k\hr = 51.69 hours = 3101 minutes
90-99 = 7,688,099 xp; @80k\hr = 90 hours = 5400 minutes
Therefore total time = ~9800 minutes.
Assuming 70 prayer
Each prayer dose will increase 2.24 minutes of prayer.
Therefore you require 4352 doses of Ppot, which is 1088 prayer potions (4).
That's for 70-99 str; or I might be completely retarded and everything I've done is wrong.