After 25 years of marriage, a 4 month break may not be a bad thing.
When you consider most marriages (50% or more- don't quote me on that, I don't have a photographic memory), never get as far as yours has, that does not necessarily mean the end of the marriage.
For a marriage of say, 5 years or less, a four month break is very significant in my opinion. But, 25 years, you know this man, and more than two decades together, is a significant loss- for both of you- if it ends. Very sad.
You note that he is travelling more than he has in the past 25 years, and this seems to be happening while he needs his 'space'. To me, that is suspicious. While he may stay with relatives when he is not travelling, when he is travelling, it is only him. He's not going home every night. Why do you seem so adamant that it is impossible for him to be having an affair.
Seems to me if he were unhappy with you, or the two of you were arguing a lot, or had problems financially, or you were both sick and tired of each other, you would have certainly agreed that he was just as unhappy as you were. But, you were not unhappy for any obvious reasons, yet he was. That you didn't see it coming, also seems suspicious to me. He's kept his unhappiness from you- why do you think he would not want you to know.
There has to be a reason. After a successful 25 year marriage that suddenly has one party put on the breaks and take off- what could it possibly be- any ideas?
During the four months you have been apart, do you talk, text, email, phone each other? What do the two of you talk about to address this absence, and have you discussed what could possibly be done to figure it out in order to get the marriage back on track?
You know this man better than any other single person on this planet. Your question , "how can I know if my husband will come home", is not going to have an answer until you demand one.
I agree too that because you were blindsided by this, you might want to consider speaking to an attorney, before you go to buy groceries or put gas in your car, and realize that all your accounts have a zero balance.
At least protect yourself while you figure this out. Don't wait for divine intervention, because it won't happen. Take care of yourself, and be prepared for the worst.