opportunity seldom knocks twice
rarely
I can't remember the last time
seldom
uncommonly
occasionally
nonrecurrence
infrequently
once in a blue moon
casual
unresumable
infrequent
ever
seldomly
come and go
sometimes
asynchronous
accidental
nonfrequent
nontimed
occasional
untimed
seldom or ever
nonrecurring
recurrence
return
unoccasional
slow
sporadical
temporary
fugitive
momentary
momentaneous
fleeting
go narrow
nongradual
once in a way
receding
fadeout
frequent
quasiperiodic
secular
impersistent
on occasion
at times
now and then
catch-as-catch-can
once in a while
aperiodic
contemporary