It is high time you started a new business.
What do you say about the mix of tenses in the Spanish translation? Going from English it sounds logical, but I think “lances” would be more correct. 
It should be noted that the Tatoeba sentence was contributed by a native English speaker living in Mexico.
Yeah, probably. But I’d change the tense of “ser”, instead of “lanzarse”:
Ya era hora de que lanzaras un nuevo negocio.
Changing “lanzarse” (Ya es hora de que lances un nuevo negocio.) would rather translate into something like “It is time for you to start a new business”.
But still, I think it’s a recommendation about what to do now or in the near future.
Isn’t that what the English sentence is saying already?
To me, that is something to say looking back at the successful launch of the new business.
hmm… I understood the English sentence as the second option so, as you said, after launching (successfully) a new business. But if that is not the case, and it is a recommendation about what to do, then “…es hora de que lances…” is the right one