client2014,
generally, as long as he/she wants and you both can afford. But he/she has to return to Canada for the hearing (although I've heard about hearings held by videoconference from another country, but only in some exceptional circumstanses). There are some limitation though:
If your partner is a Canadian citizen - he/she must show that you both intend to live in Canada after appeal allowed and PR granted. Otherwise this can make another reason for refusal...
If your partner is a Permanent Resident - same applies, but he/she has to reside permanently (which doesn't restrict going for vacation or something like that) in Canada after appeal allowed and while PR application is being processed. And as long as he/she complies with Canadian residency obligations (2 years out of 5).
In any case, this is measured in years, not months.
And of course make sure he/she will be able to receive and respond to any communication from the IAD.