A colleague asked to check in with me every couple of weeks. I would like to respond that it would be wonderful if they checked in ever so often. But, I don't believe that sounds correct. How should I phrase it?
Ever so is a rather archaic way of indicating degree, as in "I like picnics ever so much!" It is something you might hear Dorothy say to the Scarecrow in The Wizard of Oz.
If you hear it used to mean every so, the speaker is probably dropping the -y from the end of every. What you're hearing is probably:
I've heard the -y elided in this way in the southern and western regions of the U.S.
ever so often
The phrase ever so simply means very. When used with often, it emphasizes the frequency slightly more than very does. Compare:
In the second example, the speaker subtly indicates his/her slight irritation or delight in the fact that "he" visited/called/stopped by to see the speaker so frequently.
every so often
This phrase means occasionally. It can be easily substituted with from time to time, sometimes or every now and then. Examples:
Every so often is more commonly used, I think, though I have this hunch that the meaning is sometimes confused with very often, which is just the opposite. Your sentence probably did not sound correct to your ears because one rarely hears ever so often these days, save in literary and poetic contexts. To analyze your response a bit further:
It would be wonderful if you checked in every so often.
That's great, but it would be wonderful if you checked in ever so often.
Honestly, since your chief aim is to communicate, I would suggest you avoid ambiguity. Hence, I would recommend using every so often or, better, another adverb that clearly expresses your thoughts.
NB: I confirmed my definitions with the New Oxford American Dictionary, 2nd Edition (installed on my mac) and the Cambridge Advanced Learner's Dictionary.