1. This is an example of an expression
that has altered its spelling over
time. "Not for donkey
's ears" means not for a very long time and refers to the length of a donkey
's ears. However, they gradually stretched from
" and turned "ears" into "years". The meaning of the expression
is the same, if it does
make rather less sense
than the original.
2. Because donkeys
tend to plod along slowly, they take a long time to complete a task or journey. Therefore, a donkey
a long time to elapse.
I haven't heard that one for donkey's ears.