I would request that it be written if your ex moves in with anyone or anyone moves in with her that is not a relative and is of the opposite sex for a period of 3 months that alimony be terminated. As the two of you are attempting to broker a deal it can't hurt to make that request. If she agrees the judge cannot override it.
The problem with cohabitation is that you have to prove economic dependence has been transferred from your alimony payment to the party with whom she is living. It's very difficult to get your hands on another party's bank statements, credit card invoices, have a PI follow her around, etc. Some folks go completely underground with cash paid jobs, use a check cashing vendor so as to not have a checking account, etc. You don't really want to have to go through all of that. It's expensive, stressful and did I mention expensive?! A decent person would agree that if they are in a relationship that is serious to the point of living together, that they have moved on not just emotionally but have transferred all expectation of financial assistance (presuming it is needed) to another party.
You may also want to have language that addresses not just a change in your financial circumstances (no bonus, unemployment, etc.) but also a change in her financial circumstances. Typically, a 20% change in income is recognized as creating eligibility for a review of the amount being paid. Has she recently completed schooling or changed careers where it is possible that her income will go up by a good amount in the near future?
As for amounts - generally speaking appr. 33% of the difference in income is awarded as alimony. (Judges will deny that they use this percentage but it's been proven time and again throughout the country). However, keep in mind that alimony is calculated AFTER child support is deducted from your salary and PRIOR to calculating the difference in salary. You also need to take into consideration the assets you will be dividing such as pension, 401 accounts, IRA's etc. You get some of hers, she gets some of yours - although with the income disparity it may be simpler to deduct your portion of her earned assets from the amount you need to turn over to her. Don't just presume everything will be split 50/50. Negotiate it.
Same thing with debts - if you are taking on more than 50%, you need to look at what assets you are turning over to her to decide if the debt allocation is fair. As Bali pointed out using the figures that you provided - you will only have $1,000 more a month than she does after CS and SS - thus, does it make sense that you assume a full 10% more in debt than she does? And the numbers that Bali used were gross, not net.
You need to look at not just the income disparity but also the difference in tax brackets between you, what percentage of taxes will you be paying once divorced, who is assuming the cost of the insurances for the children, who is providing funds for the "extras" like camp, prom, sports registration and supplies, dinner dances, sweet 16 parties, haircuts, cell phones, book fairs at school, school lunch fees, trips, uniform costs - the list goes on and on. I don't know how old the kids are but you have to think about not just now but 3 yrs from now and so on.
Are you going to have to obtain a life insurance policy naming the STBX as the beneficiary? Will you need to obtain a separate policy to cover your alimony payment in the event you pass away? Are any of your current insurances bundled for a cheaper rate that you now will lose? Will you be paying higher premiums for anything once you are single versus married? Will your rental/mortgage costs increase?
Attorneys would like you to just look at the difference in income and work from there. So would the courts. However, disposable income is affected by numerous items. Sit down, and start tabulating what is going to be affected, what you will be paying, etc. and see if you can afford the proposed budget or not. I've given you a few items to investigate to get you thinking and on the right track - there are plenty more. Only after creating a budget can you begin to determine what is do-able for you not just today but a few years from now.
As for the length of time. Well, that varies by state and by courtroom/judges. With a 13 yr marriage, it may be considered a long term marriage which would generally equal either half of the length of the marriage in some states and in others it would equal permanent alimony. Negotiating it to a third of the marriage is most likely an improvement than what you would be ordered if you went to court.
Good luck to you. Please post again if you need further assistance and I'd really appreciate it if you would provide us with an update when all is said and done.
Smokes!! That's really crazy. I'd find a job in TX, transfer and then file for a divorce!!