be + (like) chalk and cheese
.
while + old chestnut
.
this time last year + chase rainbow
.
how long + to pass with flying colours
.
for + a cakewalk
.
before + keen as mustard
.
last time + a can of worms
.
since + bookworm
.
after + as useful as chocolate teapot
.
ever + a cast-iron stomach
.
last week + to rack one's brain
.
when + children's play
.