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