How much mmmm should be in hmmmmmmm

Solution 1:

Hmm. The Macmillan Dictionary has a listing for hmm, but not for hm or hmmm.

Oddly, h'm is listed as an alternative. However, according to Google Ngrams, the prevalence of h'm has fallen off sharply since the 1940s. (I don't think I've ever seen h'm in print.)

Solution 2:

Interesting... when writing I use 'hmmm' as it appears more obviously a longer pause for thought than just'hm' - which looks like a misspelt 'him' - or even hmm. It's purely a matter of appearance. any more m's would indeed look false and in my opinion would add nothing to the conveyance of meaning.

As to 'Shhhh', it depends on the character saying it and what they are trying to achieve. A quiet 'Sh' or 'Shush' between two people is different from trying to persuade those around you that they should keep quiet. In that case a larger number of h's appears to be acceptable - up to a point. That such a point remains undefined is one of the delights or aggravations of colloquial English!