Castle Clash: Is there an optimal time to use gems to buy heroes?
Solution 1:
As far as I know, there is no optimal time to spend gems, there may eventually be an event that might help you with that but as far as I know, the answer to your question is no.
Solution 2:
There's a discover the treasure event now and then. You will get a chance to win random awards by spending 2k gems. Personally I got 3 cupids out of the event. Now hero collector is the new thing, but you gotta save around 60k gems to worth a try.