Mobi bubbles

The clear cartridge of my Tandem mobi occasionally develops the odd air bubble. It seems likely to me that my previous pumps also sometimes had bubbles in their cartridges, but I’m only seeing them now because mobi’s cartridges are transparent and not enclosed.
My question is, does anyone have a rule of thumb for how long to hold down the button in order to push a bubble out through 32 inch tubing? When I’m holding down the button I can see the bubble as it makes it’s way out of the cartridge, through the connector and up into the tubing but once it’s in the tubing my eye can’t follow it. I hold the button for a while to make sure it’s out, but I’m loathe to waste insulin and hasten my next cartridge change. Not sure if I’m over or undershooting when it comes to the bubble.
I could unscrew the tubing, push out the bubble from the little connector and reconnect, but I’m worried that I’d just be creating a new bubble that way.

I put my mobi so the tubing is down and then bubble will rise to top away from where it enters the tubing