The dates were announced today, and they're roughly monthly.

From the BBC site:
http://www.bbc.co.uk/news/uk-england-london-11083153

"Maintenance and engineering staff will walk out for 24 hours at 1700
on 6 September, 3 October, 2 November and 28 November.

Other workers, including Tube drivers, signallers and station staff,
will strike for 24 hours from 2100 on the same dates."

I'm not sure if maintenance staff walking out will have any immediate
effect on services, but last time the drivers went on strike, I recall
services began to wind down a couple of hours beforehand. Presumably
services won't get back to normal until start of traffic on the third
day, so it's rather more than a 24 hour stoppage.