It is known that even a Buddha, who has practised for many many
lifetimes before, attains enlightenment after at least 7 days of
meditation. Shakyamuni Buddha took 49 days.
For normal people like us, it will usually take years or decades or
lifetime or more lifetimes of practise.
Also, it is easier to get a glimpse of nirvana (when you reach
sotapanna/stream-enterer stage you will have a glimpse of nirvana),
than to eternally abide in nirvana, which is only possible if you
are at least an Arhat. There are people in this forum who have
attained many glimpses of nirvana, but so far, in Singapore I have
not yet known of anyone who abides in nirvana permanently. Or I may
be wrong.