There is a bus going from the village of Trebujeni to the town of Orhei. It arrives at 3:30 pm from the bus stop near the village hall (in Moldovian called Primaria). You need to walk from the Monastery from Orcheiul Vechi to the village of Trebujeni (about 30 - 40 minutes by the cliff), take a bus to the town of Orhei and there are lots of buses going from Orhei to Chisinau.
before you'll get to Trebujeni, or when coming back from Trebujeni!
I would advice you to stop for a snack (which you will bring with you) at Ivancea Lake. Really worth stoping, there was a summer camp once (probably exist now too) and you can stay there to stop for a while before your new adventures!