Um, cheating changes the WHOLE dynamic. In order to have a "polyamory" (see multiple LOVES relationship) one must be able to LOVE one person first. Cheating is not loving.
If someone is cheating they aren't loving themselves-someone who loves themself is able to be honest and true about who they REALLY are. Cheating is in and of itself being dishonest and untrue about who you really are. If you can't love yourself-you can't possibly love someone else (which is necessary for a monogomous relationship) and if you can't do BOTH of those it's IMPOSSIBLE to truly love yet more people.
You need to address truth, honesty and realism in your relationship before you can even START to address trust and polyamory inthe relationship.
How long that takes depends entirely on how well you each handle that responsibility.
"Love As Thou Wilt"