No they don't have an expiry date. You can only forfeit the 'partly paid (pp)' shares if you fail to make the CALL payment on the closing date and time set by the company.
There is no set time-table as to when the company can make these CALLs (unless a time-table was set out in the prospectus).
In this case I believe, you can if you wish, pay the outstanding monies to convert to fully-paid (fp) shares. However, unless you have some specific financial reason to do so it would not make rational sense since the market for the fp is < pp+the outstanding amount [ fp =21c :: pp = 3.8c + 24.99c = 28.79c ].
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