How long is it good for after opening?

The container has an expiration date on the cap and also says "Keep at room temperature, refrigerate after opening, shake well before use" Mine have been in the fridge for quite some time and I've never worried about them being good - I just try to not have a bunch of them open at one time, so they get used up faster.

Are any of these caffeine free?

The root beer is. Not sure of strawberry.

why cant I get diet tonic ? out of stock for over 6 months!!

It started on Bill Clinton's watch when he approved NAFTA. Then American manufacturers decided to save a buck and have all of the manufacturing sent to Asia. So there were no more paying jobs. So we have been completely reliant on the worlds master - China. With the current virus situation, it has made it simple for China to hold back on manufacturing to hold our products hostage just because they want to flex their international muscle. While Soda Stream is made in Israel, I'm sure they are waiting - just like everyone else - on something from China to complete their process.