My rule is to defend when short strike touched or 200% losses. I defended about 50% of short 4400 SPX calls the other day by rolling them up to 4500 and essentially doubling down on the number of contracts. Take a look at (admittedly, I pulled the trigger early as neither of the conditions were met).

 Note that this is absolutely not the way it's taught or the safest approach. It only works if You started out around 2 to 3% of your portfolio in short premium and it's your first roll. Its less stressful from a margin perspective in portfolio margin accounts... But there's a relatively high bar to open one of those. 

The more standard approach is to roll out a month and up.