There's no meaningful answer because the credit reports of two people could differ enormously in the things needed to improve them (and associated timeline).
Suppose Bob and Tom both have bad scores and they both are aiming for a 760.
If Bob has a new chapter 7 bankruptcy, he'll need to wait ten years.
If Tom's bad score is caused by ultra-high utilization and having no loans, he can fix that in a couple months.
Your best bet is to find a few other members on the forum that started with similar profiles to yours and see how they were able to rebuild and improve their scores and over what period of time. CGID is correct that profiles can be wildly different.
On my thick/aged/low utilization file, I started with 4 dirty accounts and a 619 score. It took me about 18 months to clean those 4 dirty accounts (via GW requests) which took all my scores into the 825-835 range and then another 6 months of tweaking some things and aging to hit triple 850's.
In a nutshell, basically an apology and a request for forgiveness in the form of a credit report adjustment. You can read a lot about GW letters in the rebuilding section. There's also a thread on the Saturation Technique, a method of sending out a ton of letters in order to raise your odds of a favorable result.