"Which František are you waiting for?"
Translation:Na kterého Františka čekáte?
Hard to say. It's more of a question of customs and manners, and even natives have sometimes problems to discern when it is appropriate and when not.
A few pieces of general advice that I could give you, stick with 'vy' at first, don't address someone using 'ty' unless you shake your hands and there's some kind of an introduction. Then it depends on the situation, your/the other's social status, position of authority, age, gender etc.
To illustrate it a bit: using 'ty' with newly-made friends of your age in a pub/café is perfectly appropriate, the same for your new roommate, classmate etc. Using 'ty' to address the mother of your SO/your new boss at work after you've just been introduced to her would be seen as rude. The same with using 'ty' to address your much older female co-worker, unless she has given you leave to do so. You wouldn't use 'ty' to address your customers or clients as well.
When in doubt, ask or offer using 'ty'. But you should know that the person with the 'higher status' offers first.
To confuse matters further, we sometimes use first names with plural 'vy' forms. That's kinda semi-formal. Your boss might address you like that. Or your teachers.
It's often pretty bad when a Czech uses "ty" inappropriately. It's a faux-pas. How bad it is also depends on the addresee's disposition - some take it with more humor, some take offense.
But when a non-native uses it wrong, it will most likely simply be ascribed to "he/she doesn't know enough Czech to know the difference." People are generally aware, for example, that English doesn't have the ty/vy distinction.