Valid QDRO never filed - how long do I have to wait to collect my portion of pension

I was divorced over 25 years ago, for a relatively small pension ($350) month I was suppose to collect 40% at age 65. QDRO was originally rejected and never corrected, then divorce was finalized.

Its not worth hiring an attorney - the plan administrator has attempted to contact my ex-spouse with no response by her.

What/who determines how long I have to wait to receive my portion? 

Are you the participant or the Alternate Payee?   I assume you are the Alternate Payee?  If so, the Plan Administrator will not pay you without a valid QDRO.  If the QDRO was originally rejected and never corrected, then the plan has no authority to pay anything to you.  The Plan Administrator is not bound by the divorce decree.  There must be a QDRO in order for them to separate the participant's benefit.  

Generally a plan will have QDRO Procedures that determine what happens in this situation.  My experience is they will give the AP a certain amount of time to produce a valid QDRO (like 180 days), if nothing is provided, they will just go ahead and pay the participant the entire benefit.   That said, it sounds like the participant isn't requesting anything at this time, so likely nothing will happen until then.

Short answer, you will need to retain a lawyer to draft a DRO, which you then submit to the Plan Administrator for approval, making it a QDRO.  A life annuity of $140/month starting at 65 is worth about $20,000 based on standard life expectancy.  Your call if that is worth the cost to hire a lawyer. 

The law allows a "reasonable time" for correction of any defects in a proposed QDRO. 18 months is a time that is often mistakenly considered to be the reasonable time, but 18 months might just happen to be reasonable under the circumstances. If the reasonable time expires without some action or further notice, the plan proceeds as though nothing had been submitted.

If you are the plan participant and it has been years with no intervening action, the pension should start with payment in full. It would be a breach of fiduciary duty for the plan to delay start unless there is some order applicable to the plan for delay. What that might be, I cannot imagine.

If you are the would-be alternate payee, then see Effen's post.

I am the participant and at age 65 I requested payment. 

Maybe using a QDRO service for a fixed fee might be better option than hiring a family law attorney.

Who is responsible for preparing/submitting the DRO?  Usually, it's not the participant.

It is heartening to have someone do the right thing and pursue the outcome agreed (mandated?) in the divorce even though a more favorable outcome is possible now, possibly at the “fault” of the former spouse.

Especially if if the former spouse was charged with prosecuting the QDRO (which is usual, as noted by david rigby), consider having your former spouse share the cost of however you go about obtaining a QDRO that reflects the original agreed/mandated terms.