If you use the free version of Hotmail, Yahoo! Mail or Gmail, each does have a specified amount of time before the account is deemed "inactive" and all mail deleted.
It is important to know this stuff if you’re not going to login to your free webmail account for an extended period of time.
Time before account is deemed inactive and you lose the mail: 30 days. And if you don’t login to the account for 90 days, the entire account is deleted.
How to extend this time: Upgrade to a Hotmail Plus account. Extends the 30 days to 12 months.
Time before account is deemed inactive and you lose the mail: Four months. It is unknown whether Yahoo! permanently deletes accounts for extended inactivity, but the answer is most likely a no.
How to extend this time: Upgrade to a Yahoo! Mail Plus account. Like with Hotmail, it extends the time to 12 months.
Gmail / Google ID
Time before account is deemed inactive and you lose the account: Nine months. Like with Yahoo!, it’s unknown whether Google permanently deletes accounts for extended inactivity.
How to extend this time: No option.
It is true you could purchase additional storage. Minimum cost is $5 annually for 20GB of space. The additional storage purchased applies to your overall Google Account and not solely to Gmail. This means any Google service, such as Docs, will also utilize the purchased space.
It is important to note however that purchasing additional space does not give any guarantee your Google Account will stay active after an extended period of inactivity. It is stated plainly by both Hotmail and Y! Mail that all "Plus" customers are only required to login once a year; this is one of the features of becoming a Plus member. Google’s policies on the other hand are a one-for-all whether you have a paid account or not.
Put another way: Don’t bother with the additional storage unless you need it, and login to your Gmail account at least once every 9 months to avoid having it deleted.
If you’re wondering who would ever leave a Gmail account dormant for that long, there are more than a few people that use Gmail only as a spam filter. Once set up, these people rarely login to the mail service afterward.
As long as you periodically login to your Gmail account, you’re in good shape.
Does using the instant messenger service for a webmail address count as legitimate activity on the account?
The answer to this question is a conditional yes, and the condition is easy to meet.
All you have to do is use an instant messenger client that periodically checks the mail for the account, even if you don’t use the mail. Each check of the mail counts as a login, and therefore counts as activity so your account won’t get the axe for inactivity.
Alternatively, you can use an in-browser email checker if you don’t feel like using an instant messenger client. Using Gmail as an example, any "notifier" app for Gmail will perform this function adequately.