It should take roughly a month. If i might make a suggestion, use ruskis autofighter. Its never let me down. I got from 65-85 str in 3 weeks. No ban on my account. And don't bot more than 7 hours a day.
I made roughly 960-980k xp a day using Ruski's. This not only gets you Strength, but HP as well. (i already had 99 attack, so that may have effected the XP-rate)