There is no minimum timeframe unless it was inserted by creditors at the meeting. Talk to your IP and explain why and how you wish to settle and they can advise if they believe it would be acceptable.
I made a full and final offer after just one monthly payment - the original return to investors was to have been 18.5p/£ and the full and final offer was 14.5p/£.
Only three of my creditors voted on the F&F (the smaller creditors) and all three accepted so it was passed. I settled and thankfully my IVA is now over.