Switching filters - how long until new one is mature ?

Hi,

I have a 48 x 12 x 15" tank which is home to my three large goldfish.
I am moving from a Fluval Biolife to an Eheim 2213 as the Biolife is
just too noisy inside the tank (for the fish - I'm trying to give them
as natural an environment as possible).

I currently have both filters running, but need some idea of how long
I need to do this before the Eheim is sufficiently mature to run the
show alone.

The problem I've got is that it's a one-way decision - if I pull the
Biolife too early, it'll take a couple of days for the ammonia to
start showing - by which time the Biolife will be bacterially dead and
useless to put back in !

Unless anyone knows of a way to keep the bacterial colony alive with
the media out of the filter ?

Any ideas anyone ?

Many thanks,

Kev.