So how often do you guys or girls swap your key fobs over to keep them charged.

I personally swap mine approximately every 3 months and they seem to be quite happy with that in the two years I've had them.
I don't think the handbook states a time, just to periodically swap them to keep the charge.

Once a fortnight, Mrs Ad has one, I have the other, it gets used every other weekend when it needs diesel

No regular schedule. I just try to remember to use it occasionally on a long run.

It's interesting that some have polled more than 6 months, I just wonder as to the maximum that a fob will stay in a usable state before refusing a charge.

my old car the fob wasn't changed for over a year and the other fob worked fine, i think i am on to about 8 months on my current fob and will be swapping over soon as i can be arsed

Periodically to me means once a year  Former Mod/Member, with the most post & Chicken George Arch nemesis

Manual says minimum of 4 hours every 6 months, I swap every month, I have a keyring on the one I'm not using, 1st day of the month I swap it over, don't have to keep track of anything that way. 
Hung like Einstein, smart as a horse.

Mrs D uses her one every day, I use my one probably two or 3 times a week.

When I got my new key programmed last week, I was talking to the receptionist about non-use of the fob. She drove a FL2 and argued that there was a replacable battery in them and that they don't charge up! She went out to the workshop to check and came back all sheepish. "Well, I've worked here 7 years and I never knew that - I'd better dig out my spare when I get home tonight"!  Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

It's highlighted the fact that many of you share the car
My wife has her own and has never sat in my driver seat so the responsibility of swapping the 'fob' is mine when I remember to.

Like Pinky, I only change Fobs when I remember.
Got a longish drive coming up so I'll use the spare key then. Afterwards it'll go back in the draw.

The 3 memory seat settings depend on which key you use. Thought once I had up my drivers seat position until I worked out I was using the spare Fob.

Are you definately sure on that - we set our seat positions before putting the fob in? So do I need to set our memory seat positions with each fob? Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

Yes and yes, if you want the same seating positions for each Fob

I have one key and the wife has the other, they are both used most days.

Richard FL2 07 TD4 GS Auto
Ex 02 TD4 Auto

I voted for once a week, but it's actually much more regular than that.
More like almost every day.
I must have OCD

Or more like a fear of a dead fob  Another member of the failed FL2 clutch/DMF club, twice.

Mrs g has all kinds of junk on her fob , keys for her mums houses sisters house etc etc so I use the second fob with not even a keyring on it At work
At home

"I can't always believe facts I read on the web" - Charles Dickens

winner by default of the tractor vs caravan race

One of mine was broken when I got it, someone having forced out the emergency key and broken the retaining tab. Only noticed when the main part of the fob dropped off the ring - lucky I noticed. So now that one is held together with a bit of bright red tape, which means I remember to use it occasionally.
Which is a very longwinded way of saying make sure the spare fob looks different - that will remind you to use it from time to time.