Toyota Repair: Replacing struts, struts, shocks

Question
How often and what is the cost range to replace all four?

Answer
I don't want to sound flippant, but, replace them when they need them. There is a schedule set in your owners manual, but, you may need to change them more often.
Struts should be replaced in pairs, front sets and rear set.
How much?!? Depends on your vehicle as well as the aftermarket (Monro, Gabriel, etc) struts and styles available.  
You can DIY if you've the tools and the patience.

Hope this helps.

Vince