It kind of depends on how you view the time frame. If it's not meant to be an ultimatum, but a time out, it might be helpful.
With my H, he was tormenting himself with trying to decide. Every day his family (and pre-DBing, me) was saying 'What are you gonna do? What are you gonna do?" He couldn't move onto important questions about what he truly wanted in life, what went wrong etc with everyone asking him to decide on the ending.
The day I said gently "Let's take the summer and figure ourselves out" it's like his heart opened up. He could stop trying to decide (and naturally, constant pressure had been making him want out, so the relief was a bonus for me) and suddenly could focus on the real problem (not just the stay/go issue).
He now knows that I'm not looking for an answer every time he sees me and our R is getting more and more friendly every contact. He now speaks in "we" while I still say "I" or "You". He is no closer to coming home, but he seems to feel at ease around me now.
It helps to ease guilt and pressure, which are two of the biggest factors in their resistance to the M ("How can I stay married when this is all I feel around her?")
I guess a time frame has no consequences, unlike an ultimatum. It's just "how about we do the work we need to do, and we'll revisit it in a few months".
It's hard with pressure from families and friends who make you feel humiliated for not putting your foot down about OW. That's how I felt and I gave him an ultimatum and now he lives with his parents. I regret it because I miss him. But I find that if you ignore OW's existence and approach it as the need to work on yourselves as individuals and to see how it will change the R people are more supportive.
