Is "I've told you at fewest ten times" grammatical?

Solution 1:

The question seems to deal less with the 'less/fewer' issue than the 'at least/at fewest' one.

'At least' is a restrictive particulariser. These refer to a specific number, a subset of a larger quantity, and as the lowest limit of that larger quantity, seen as a magnitude. 'At fewest' is illogical, as 'fewest' is a superlative, which implies 3 or more things being compared. It refers more to the objects being compared more as multitudes or specific quantities, rather than a magnitude of a large, unknown quantity.

I would therefore argue that 'at least' is correct in this case; 'at fewest' is not.

Solution 2:

The British National Corpus has 25,000 incidents for "at least", none for "at fewest" http://bnc.bl.uk/saraWeb.php?qy=At+fewest&mysubmit=Go

Google Ngram confirms this fact link