Whether you are petitioner or respondent is not relevant to your eligibility for spousal maintenance. The length of your marriage is the first of several hurdles to any post-divorce maintenance. Ten years is the minimum length of marriage for post-divorce maintenance, though you may still be eligible for temporary spousal support while your divorce is ongoing.
Any award of spousal maintenance is dependent on a host of factors, so you will want to consult with a family law attorney to determine your eligibility.
You will need to consult a lawyer familiar with family law in Texas. The short answer is that there is not a written formula. In fact, spousal maintenance in Texas is very hard to obtain. I haven't researched it in quite some time, but I recall that it is reserved for cases in which the departing spouse simply has no means to provide for his or herself. Merely because a spouse has not worked (and is stay at home parent) is not relevant to the analysis. The question is whether you are capable of earning a living. Lastly, there is an outside time limit that puts a cap on the length of time such an award can be collected. Short answer: consult with a competent family law lawyer soon.
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