Given Google's previous release schedules, one can almost safely assume that the next 10" Nexus Tablet won't be release until at least this fall (think Christmas shopping season)...but as nothing to date has been announced, no one can really say whether or not it would even be later than that.
Many speculated that a refresh of the Nexus 7 would be announced during Google I/O a week or so ago, since that marked the year anniversary of that launch...and it didn't happen.
To me, it would make sense that while their Nexus line of phones gets a yearly refresh, the tablets can go a bit longer between upgrades, since tablets generally have more robust hardware and aren't an item people tend to upgrade as often.
Again, this is all just speculation, but I don't think that buying a Nexus 10 now would be a faulty investment.