There is no such thing as a "scheduler issue" in JIRA Core (or JIRA Software for that matter, and though I'm less familiar with JIRA Service Desk, I haven't heard of it existing there either). Is this in reference to something provided by the TheScheduler plugin? If so, then you should indicate that in your question.
In any case, you are correct; it is not possible to set an interval of 11 weeks using a single cron expression, as cron expressions work kind of like regular expressions – they represent a pattern and the job executes when the current time matches that pattern.
Plugins can also schedule interval jobs, so if you currently only see cron schedules offered, perhaps you can ask the plugin developer to support interval schedules as well.
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