No. There are various limitations that may apply depending on where you are in the divorce process, and if post-judgment, whether the court retained jurisdiction over support. But there is no actual statute of limitations.

There is not specific time, but there are several limiting factors which include time since separation, number of years married, health, age, job skills, education, whether she has sought employment, etc.

She can ask for support pendente lite (pending the divorce) at any time up to trial. At trial, if she made support an issue, the court can fashion a support order in its judgment, that will contain the terms of how long support is to last, and the factors that will cause support to terminate.

If the trial court does not order support, and terminates its jurisdiction to award spousal support, she has the same time limits to set aside the judgment as any civil judgment, such as moving to set aside a new trial (15 days from notice of entry of judgment), or 60 days to appeal from notice of entry of judgment. If judgment has been entered terminating her right to ask for spousal support, and the judgment has become "final" it's too late for her to ask for spousal support now.

The court that has issued a spousal support order has continuing, exclusive jurisdiction to modify the spousal support order throughout the existence of the support obligation. (Fam. Code, sect. 4914.)

Unless jurisdiction to award spousal support has been either expressly reserved by the order or impliedly reserved (pursuant to Fam. Code, § 4336), postjudgment spousal support is limited by the stated duration of the order. (Fam. Code, § 4335.)

I'm not sure whether you asked your question out of curiosity, or are somewhere in the procedure of going through a divorce. Some information can be found here:

http://www.kinseylaw.com/clientserv2/famlawservices/modifyorders/modifyspoussupport/modifyspoussupport.html