How long is too long to let a Testarossa sit?

Too many variables to make a definitive statement. Depends on cars current level of condition and usage level. It could be on the cusp of problems right now and a couple of weeks will cause issues. If it is unavoidable just make it as short as possible.

 

6 months - no sweat
1 year - pushing it

 

As far as the fuel goes, a mate of mine has very large car collection, and what he does is he drives the car being stored until it is very low/ almost out of fuel and puts 10-20 litres of Avgas hi octane in and goes for another lite drive and he hasn't had fuel gumming up the works. Never done it myself but i don't let my cars sit.

 

IMHO:
Tires die whether driven or sitting. It's sad but true. Kind of general rules of thumb: 4 years, they start feeling 'off.' 6 years are appreciably bad and unsuitable for spirited driving. Some would say, "No driving"! I've thrown away so many new-looking tires that I've lost count. Fresh tires feel great. New tires just broken in/scrubbed in after one or two hundred miles - even better!

Something (else) to consider with your storage plan and rejuvenation after hibernation.