In my experience extending 1500 hours and having talked to may other extender users - plan on 1500-2000 hours under tension to gain an inch - so a lot of your question depends on how many hours per day you are extending. At 2 hours per day, you would be looking at 2-3 years for an inch, or roughly 6-9 months per quarter inch. At 4 hours per day these estimates are halved.. And so on.
I extended for 10 hours per day for 90 days, then another 60 days at at around 6-8 hours per day and gained .8" in those 5 months.
Hopefully this will give you a rough estimate of what to expect -though also remember everyone reacts a bit differently.