Cohabiting and spousal maintenance

How do I ensure that once OH starts cohabiting with new partner, I can stop paying spousal maintenance?

I read it's "hard to prove" cohabiting and has to be "long enough"...what guidance is there?

Don't want to be paying for OH whilst she has new guy to do that for her.

You could have it written into the court order but it would have to be for an extended period like six months or a year before the court would accept it being there.

As you say though it is almost impossible to prove. You’ll probably find the new bloke just stays over a lot, or even all the time, and still has his own address. Unless someone is added to bills etc you’ll not be able to convince a court.

You’re best bet is to avoid spousal maintenance or pay it for a limited set time period which will be the courts preferred options anyway.

Probably can't avoid spousal as she's never worked and will need "support" to get back into the workplace.

Spousal maintenance will be limited to 3 years max but still, if they cohabit immediately then I'd like that stop within 6-12m, as you can imagine.

Presumably your income is sufficient to pay a greater level of SM than she would get in benefits? If it isn't, benefits would be the first option. If it is - and you can still meet your own reasonable needs - SM is more likely. It is normally time limited say to allow her time to train and develop her employment prospects.
You could seek a cohabitation clause but I wonder if the court would take the view that SM is about her becoming independent regardless of any new relationship? You would have to get the court's approval to any changes in SM. If you did apply for a variation you'd have to show her needs have changed e.g. if she has a new partner who is covering (or capable of covering) living costs which will be reduced if they are shared.

Hadenoughnow

I don't know what level of benefits she could get, so I've no idea how to answer that.
My income is quite good (100K) so I suspect the answer is yes.
My budget shows me providing some SM but the point here is, when she has moved in with new guy and they have been together for a reasonable period of time (subjective, I know) then the SM should stop if new guy can support her (similar salary), otherwise that's unreasonable and I would, in effect, be supporting them as a couple rather than just her.