There's no official answer on this. VMware employees are not allowed to tell you even if they would know and partners have to sign NDA's to not disclose any dates or future features.
However if you look at the following document (which I just updated for the history release on Fusion 6 and Fusion 7) then when looking at the past, new releases appear to be annual and around end of August, begin of September.
Please note that the past is not a guarantee for the future and that I have NO internal knowledge about when a new version is going to be released.
So take the details with a grain of salt.
As there is not even a beta out at this moment, I'd say that a new version is not even close.
If you would buy within 30 days of release then there's usually a free update and otherwise you can still upgrade for an update price.
Hope this helps,