Agree with most of these comments. Specific requirements depend on your location, and business area.
For us archiving was as much a way of storing business information for posterity. We have used email for less than ten years and as a 180 year old historic institution we are very interested in maintaining a record for future access.
Others here in the states try to reduce the term of storage in line with policy to limit liability (to limit the cost of potential legal discovery requirements). But we just have it all.
When we moved to a SaaS vendor, the required extra storage only costs an extra $25/mo. ...We are a small company!
I will say that not all SaaS archiving services are alike. Our first try was a disaster - which I bailed out of to a larger vendor with competitve pricing. It gets us top notch spam filtering as well. I couldn't be happier.