A four or five year gap between when Ahsoka and Sabine last saw each other is reasonable. It doesn’t seem like Sabine trained for long given her stubbornness and Ahsoka’s hesitation to become a master after losing her own to the Dark Side. If Sabine trained for 1-3 years and then they spent a couple more apart, the math mostly checks out. That gives Ahsoka time to make her solo appearances in The Mandalorian and The Book of Boba Fett, and explains why she is so insistent on not training Grogu herself.
But if the redone scene in Ahsoka isn’t just an homage to the ending of Rebels and actually is trying to retcon the timeline, then there’s really no way to know when Sabine first became Ahsoka’s padawan unless the series specifically says or shows us via flashback.
As far as Sabine’s Force-sensitivity goes though, that mystery is a lot easier to solve. In Rebels, Sabine trains to use the Darksaber with the Jedi Kanan Jarrus and Ezra. She struggles with that training at first, growing frustrated when she doesn’t master the blade right away. Kanan gets a little frustrated too, confiding in Hera that Sabine’s Mandalorian stubbornness makes it harder for her to connect with the Force, but not entirely impossible. He says that “the Force resides in all living things, but you have to be open to it.”
When Sabine speaks to Huyang in Ahsoka, he says that her aptitude for the Force falls short of all of the Padawans he’s seen over the centuries, but that doesn’t mean she can’t still learn how to wield it. Based on the conversations we’ve seen thus far, it’s likely that Sabine once again felt too emotionally blocked to fully give herself over to the Force, whenever her training took place. But regardless of when she first started training with Ahsoka, it’s exciting to see Sabine pick up a lightsaber once again.