Not sure if this will work, but you can edit CBSubs XML file below which contains the durations.
/components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/xml/edit.plugin.xml
Can't recall if this file is available with non-GPL license of CBSubs, but should be.
It looks like you just need to add the following to relative_time.
<option value="0000-18-00 00:00:00">18 months</option>
Seams to show up in the duration dropdown, but I've no idea if the duration actually works.