Yes, they expire
i) 12 months from the last date you purchased a credit package or last date you received a compensation via credit top up.
ii) 12 months from your last completed job
(whichever of the above two has a longer expiration date)
For example: Last purchased date was on 1st Jan 2018, all your credits expire on the 1st Jan 2019.
If you top up again on the 1st of Mar 2018, all your credits will have a new expiration date of 1st Mar 2019.
Therefore your credits can infinitely last as long as you want, as long as you top up within the last 12 months.