match duration

is there a variable that tells me the length of the event?
Tennis 3, 5 sets

There is no direct information about the number of sets in a tennis match.

how could I avoid opening positions in the final part of the game?

You would need to know how many sets can take place in this particular match and check for tennis_current_set in the trigger conditions.

instead with regards to racing?
what is the variable to call to know how many seconds / minutes a race has started?

To check the time that has passed since the start of the event:

Market's Minutes since the off is ...

To get the approximate duration of the race (based on its distance), use this trigger example:

https://marketfeeder.co.uk/learn/triggers/acting-towards-end-of-race/