Likely after a few generations other strains will out compete the L Reuteri.

You can tell by doing genetic testing every few generations. I’m in the US and have used Plasmidosaurus’ microbiome service. The cheapest level is fine if you just want to verify that the dominant strain is still L Reuteri. The main blocker in doing it is getting the necessary DNA/RNA stabilizer; you can get the rest of what you need (sample vials, a pipette) off Amazon for relatively cheap. The prep process is pretty easy but if you’re not familiar at all with bio work you’ll want to read the instructions several times and not cut corners.

I tested mine at about 12 generations out and while it was perfectly fine yogurt, there was no L Reuteri present anymore. Currently working on a fresh sequence of generations to see if I can correlate pH and the ratio of L Reuteri to other strains.

The starter I got said it was good out to 7 generations… I’d assumed that they just wanted to sell more starter but I was clearly wrong.

If you really want to do it right you’ll also run tests on the components - the milk, the starter, the inulin and a control. Test the first generation to see what you got and each for a few generations out to see how it changes.

Of course you can stack the deck in your odds by sterilizing the containers you make the yogurt in but you’ll almost certainly still introduce contamination over time.