I just want to add something. Everyone here is saying much the same thing...don't allow him back too quickly, make him work for it.
I'm a bit concerned that you might let him back soon because you want him back so badly, and feel you can handle it, and that having him back sooner is better and shows full commitment from you.
But that is not the issue. You have to make him wait because he needs that time to really understand what he has done. If you let him back, you may be able to handle it fine, but that's never been in question. You have been the responsible party from the start.
Unfortunately if you let him straight back, he will never have to really face the results of his actions, or acknowledge the amount of damage he has caused. He will not have learned, and so the likelihood of his repeating his behaviour will be high.
He needs time to realise that this thing did not 'happen to him' but that he had a choice, and he, and no one else, caused heartbreak and suffering to his family.
Allowing him back before this has happened will result in a repeat of his actions, if not now then a few years down the line, IMO.
He needs to be resolved to be committed, to practise self-control, and to treat love as a verb; in other words something you do rather than feel. So one day if he finds himself bored with you or family life he will pull his socks up and work to make things better with you, not look around for a diversion.
And if you let him back too soon, it gives out the wrong message about you. Don't give out the message, 'My feelings are not as important as yours, so to save your pain I will not expect you to face up to your deeds. Your feelings are of paramount importance, so to save your pain I will allow you back in now, rather than later, without worrying about whether you have reached an understanding about the damage you have done to me. I will bear my own sadness and grief and not bother you with it while I mend myself, my family, and of course, you.'
No!
That man must learn to treat your needs and feelings with as much respect as his own. At the moment the only way he will learn to is by being made to work for your approval again.
I may be preaching to the converted here, Spook, and if so I am sorry. I am just so aware of how much this man means to you and how far you would be willing to go to make your marriage work, that I am worried you may let your heart rule your head. Think about this...tough love will work here, being soft will not. Be too soft and yes, he will come home, but he will do this again one day and then your marriage will be over. People do not learn new and better habits if others are always there to pick up the pieces for them. This time he must pick them up himself in order to see what a mess he has made.
Sorry, I feel I am ranting, but I think you are at a dangerous time and I so don't want you to be hurt any more.