training question?

okay so these are my stats-
40 att
51 str
1 def
40 range
1 prayer
25 mage

heres the question-how long will it take for me to get my str from 51 to 74ish. i will be training on rock crabs with a rune scimmi. and also can some1 tell me if i should get 31 prayer for ultimate str. okay i know it will take a pretty long time but id like to get a good estimation.