Ideally two people shouldn't be married unless they have trust.
If it's a situation where one has betrayed the other then it will most likely take quite awhile before their mate "relaxes" and trust again.
When asking for forgiveness be candid about the transgression. Take full responsibility for the (choices) you made and explain (why) if faced with the same scenario you'd respond differently .
It's not enough to apologize for what you did. You have to illustrate what you learned from doing it and why you'll never ever do it again.
Otherwise if you simply give a "reason/excuse" it means you'd do it again if faced with the same circumstances. Essentially you're blaming someone or something for (your) choices! That's not a good way to begin a fresh start.
Some couples benefit from going into therapy together. However this only works if (both) people are still "in love" and want to stay together.
Remember the timeframe belongs to the injured party and not the one who committed the betrayal. Therefore patience and understanding must be given as well as being honest and transparent about things. If you say you're going to do something do it, if you say you're going to be somewhere be there, and always be considerate.
Honesty leads to trust.
Lastly one must consider the possibility their mate/spouse may never be able to truly forgive no matter how hard they want to. Ultimately they may decide in time to walk away. Even then you have to give them credit for (trying). In the end all two people can do is their best.
There is no timeframe for having a "feeling" or "belief about someone. Trust like respect is something that has to be earned over time.