What time period before the next skill training?

If you learn HTML/CSS then after some time, just to keep in the race you have to learn or upgrade to

• Bootstrap

• then Javascipt

• then jQuery

and it goes on and on in any technology.

Asp.Net => MVC => WCF => WPF et al.

So what should be the time period before learning the next skill,

• 3 months

or

• 6 months

or

• 1 year

Please comment.