If you are old OMG certificates holder - OCPU v1, OCEB v2 or OCSMP - you're lucky! These exams do not expire.
However there are newer versions published - UML 2 (OCUP 2) and BPM 2 (OCEB 2).
In case of new OMG exams the general rule says:
"Your certification expires 5 years from the date you passed your exam.
The same or a higher-level certification must be taken prior to the previous certification’s expiration date to extend your certification."
When you pass the exam the clock starts ticking. You have exactly 5 years to choose between two options:
If you don't take any action your certification will expire. It's particularly acute if you have already passed two or more exams within one certification path. If your second or third level exam expires you need to start over :(
There used to be a "last 6-months period" policy which allowed you to retake your exam only during this time but it has been recently removed. There is also no cap on retakes.
Let's see how it works in practice - our case is the BPM 2 certification.
Once you pass the Fundamental exam, you have up to 5 years to either take one of the higher level exams (Business Intermediate or Technical Intermediate) or retake the Fundamental one when you are nearing the end of the exam's expiration date.
In fact, you should pass both Intermediate exams within 5 years so that the validity of the Fundamental exam does not expire.
Pro tip: put the expiration date of your exam on your calendar.