There will be peace when Mosul falls to Iraqi forces. ISIS has caused immense pain and suffering to the region, particularly to non-Sunnis and minorities. Although, the Iraqi army does not have the best record for human right abuses, it is still miles better than ISIS.The Iraqi army does have it's problems but it has changed considerably and Iraq will never have peace with ISIS.
There will never be world peace, because man-kind is divided. And we will continue to divide ourselves until necessity dictates it, until a force of great strength threatens us as a whole, then there will be until but no peace peace is only possible through occupations witch is not peace, as every empire has proved we as a species are always going to fight each other.
There will need to be more than a simple "fall" to Iraqi forces for Mosul to find peace. The area is too war-torn for immediate peace. The people need to come together and support a common ruler first. The people also need to find common ground and I don't think that will magically happen just because one side lost.
To think that the recapturing (liberating) of a city will bring peace to the Middle East is a bit optimistic. ISIS is still a functioning terrorist organization that needs to be neutralized if Iraq is to develop sufficient stability to bring peace to the region. This would be an important step forward, but it only presents one important battle.
No, there will not necessarily be peace once Mosul falls to Iraqi forces, but it will be a good beginning. The war against Isis and other extremist groups will likely be ongoing. We may win a battle here and there but another will flare up. There will be extremists for a long time.