How often is the fuel sampled in turbine aircraft?
Water and microbiological contamination are a threat to turbine aircraft even if water mixes with kerosene based fuels:

Industry-wide, the major threat to FQIS [Fuel Quantity Indication System]
availability is microbiological contamination of
the fuel (see FAST 38) and water in the fuel tanks.
Settled water in the fuel tanks may affect FQIS
indications, typically driving FQIS probe readings
out of limits and leading to aircraft delays for tank
draining. Therefore, a regular water drain task
is essential for smooth aircraft operations.

(Airbus FAST 42)
The reason is that both water and microbiological contamination in the fuel lead to a higher dielectric capacitance, which is used for fuel quantity sensing:

The FQIS system is calibrated for
use with jet fuel and the probes
measure from 'unusable' to full
tank capacity. However, the
dielectric of water is approximately eight times higher than the
jet fuel dielectric. Therefore, when
there is water in the proximity of
the probes, the fuel measured capacitance changes. Typically, the
probes will read a higher capacitance level than the actual fuel
level.

(Airbus FAST 42)
If only a small amount of water is suspended in the jet fuel, it can be safely fed to the engines without causing a flameout. However, water collecting at the bottom of the tanks must be drained:

All modern civil
aircraft incorporate water drainage
and/or scavenge systems to help
achieve this. The Airbus wing
design ensures water is maintained
in suspension and fed to the
engines; any remaining water is
then drained at regular intervals
via the water drain valves.

(Airbus FAST 38)
For an Airbus A320, this fuel tank draining must be performed at least once every 36 hours:

The amount of water drained is about 200 ml per week and per tank on an A340 after improvements done in 2007 (Airbus FAST 42).
Airbus recommends to visually inspect the drained water and to perform a fuel analysis at least once a year to detect contamination:

A second key element of prevention is monitoring. Often overlooked is the need to visually
inspect any water resulting from
the MPD water drain task. Drained
water and fuel will separate within
the container with the water
settling to the bottom. Both should
be clear with no particulates
evident to the naked eye. Any
'cloudiness' in either fluid indicates
possible contamination and action
is required.
Currently, Airbus recommends an
annual analysis of fuel from each
aircraft to test for fuel/water contamination, which should be considered a minimum for all operators.

(Airbus FAST 38)
Should the visual inspection show any contamination, or should the pilots notice discrepancies in their fuel analysis (see e.g. here for example calculations), or should the FQIS show fluctuations or degrading (fuel quantity indicates XXX), a detailed analysis should be performed according to this flow chart:



(Airbus FAST 38)