Besides expiring packages (personally I would set 1 year for all) I think it would make more sense to be able to limit their property posts also.
So for an example, for free package I would limit:
property posts expire in 7 days, no more than 2 property posts at one time, not able to post Featured property. (Not able to post Blog)(requested feature)
For paid I would say, property expires in 30 days, 10 property at a time, (able to blog post) etc..etc..
If property posting does not expire, we will have old property listings which are no longer on the market.
If we don’t want to loose page content, we can leave the property on the site, just mark it as “Expired Posting”
Also, to increase site participation, we can send email to agents “Your property posting is about to expire, renew your listing or mark your property as sold”
Same for packages, send email notice that their package is about to expire and to visit the page to update.