I like to store my 1995 Ducati 750ss over the winter months so just wondered how often I should run the engine ( if at all ) and if so how long for to avoid any condensation in the engine and exhaust, thanks. 
I tend to run mine once a month up to operating temp but concentrate more on bouncing the suspension, operating brakes and clutch etc. To help keep the seals healthy and not dried out!
Prefer to ride them instead!!
I don’t usually run mine up during winter, but I would think that you would need to get it fully warmed up for quite a while to avoid extra condensation building up.
I also put mine on an Optimate for about a day week, not full time.
One of our West Mids group uses a vacuum sealed bag for his which works really well, it’ a big bag which you attach a vacuum on suck, extract most of the air and seal up, keeps out all of the damp.
The older air cooled engines seem particularly prone to condensation build up in the crankcases, my 900 Monster used to be permanently white in the sight glass, I modified the breather system, took the bottle off and fitted a small K&N on top of the breather vale, much better now.
Kevin.