There is no single value or answer. Like most things in life, it depends! It depends on the condition of the battery, the level of charge in the battery, and the residual or parasitic draw on the battery while the car is parked.
I'm not trying to be evasive, so please keep reading and I'll explain how to figure this out...
This approach should work as you describe as the key element is removing the battery from the car to eliminate the parasitic draw. A fully charged battery in good condition will still lose some charge over time but it should be very small and the battery should still have enough charge to start the car at the beginning of next season.
But many owners don't want to deal with what they perceive as the hassle of removal/installation of the battery and messing around with re-setting the clock, re-authorizing the radio, etc. Thus, the battery tender comes into play with its "plug and play" minimal hassle approach.
There are various estimates of the parasitic draw on the internet. However, the actual value can vary significantly from car to car depending on the year and the options.
Thus, the best way to find this value is to measure it. Its fairly easy and quick to measure and I describe how I did so in the link. This link also describes how to calculate how long the battery can sit (without a tender) and still be able to start the car:
Spec Boxster Build
I did this because my Boxster is a race car which can sit for up to 60 days in between races and I needed to know if I needed a battery tender or if I could just let it sit and still have confidence that it would start.
Not sure what to say here. I've tried to address your questions as best I can.
No. A battery in good condition should have more then enough capacity to maintain charge between weekends (and probably for much longer).