>>527361974
The Series Battle Pass updates every major patch, so the current one with the Magitek Slasher would get replaced when 7.3 releases.
>How long would it take to unlock the Magitek Slasher?
Depends on what you pick to play. CC is going to be quicker since you more than likely will be getting through more matches than Frontlines, but your Daily Frontlines roulette earns you an additional 1,500 for the first match you play that day.
You can use this to get an idea of how much you need to play before 7.3 drops:
https://belthesar.github.io/MalmstoneXPCalculator/