Which Inspector General? Each Federal Agency has their own Inspector General, so the pay progression will vary depending on which one you are referring to.
In general though, this site is the pay scale. https://www.opm.gov/policy-data-oversight/pay-leave/salaries-wages/2018/general-schedule/
This is the GS Payscale that most government agencies follow. Salary is different by locality (city).
So pick which city you'd be working in. Generally if you are referring to a new hire position you'd be hired as a GS-7 Step 1. In some agencies after 1 year you'd be autonatically promoted to GS-9 Step 1, after 2 years you'd be automatically promoted to GS-11 Step 1, and after 3 years automatically to GS-12 Step 1.
Once you are a GS-12 Most agencies require a formal promotion process to be a GS-13.
The Steps themselves vary. For example after you are a GS-12 Step 1 for 1 year you'd be a GS-12 Step 2. It is like that for Steps 1-4, then Steps 5-7 require 2 years, and steps 8-10 require 3 years. So if you were a GS-12 Step 7, you'd have to work at that rate for 3 years before you'd automatically become a GS-12 Step 8.
This pay scale doesn't take into consideration annual cost of living raises that are approved by the President. Generally those are between 0-2% each year.
Hope this helps, but again a lot depends on the specific agency you'd be working for and where it is located.
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