you should get a different answer with a longer incubation. This will differ depending on what media type (mixo vs autotrophic), what is the history of the sample, what time of day it is etc.

I would suggest a 10-15 minute dark incubation followed by 5 minutes of far red light illumination (720nm) to oxidize the quinone pool and preventing state 2 transition in the dark.

Our instrumentation suggestions dark adaptation for at least 30 minutes.

To get a 'decent overall estimate' 30 seconds should indeed be enough. The time my old laboratory used for dark adaptation was 10-15 minutes, however the important point is to always use the time to ensure comparability of the samples.

I see a quick and easy paper in this question.  Have an undergrad student try increasing darkness times from  30 secs. to 24 hours and see what the fluorometry response curve looks like.

To my taste, Dennis has the right answer. Even if the result might not be an easy paper, your question reminds me of the tenets of "Accademia del Cimento" started by Galileo's students, among which, first and foremost were experimentation and avoidance of speculations

Thank you all for your feedback. It sounds like 10-15 minutes is a good starting point, but some validation is in order.

Audrey is right on that note. Simply doing a timecourse of longer and longer dark adaptation and determining the time that gives you the best Fv/Fm is not really the best way to go, In my opinion.

There a number of factors that feed into Fv/Fm and a big one is NPQ, or non-photochemical quenching. Qe, Qi, Qz, and Qt all feed into NPQ, Dark adaptation alone will relax Qe and Qz, while Qi (photoinhibition) requires time to synthesize new PSII and a proton motive force to enable the insertion of new proteins into PSII (far red light should do). Qt (quenching due to state transitions) requires an oxidized quinone pool and is where the far red light (720nm) comes in handy because it preferentially excites PSI, draining the quinone pool. 

The time of day and media are also factors, as well as how the cells are handled. Try to avoid experiments within the last 25% of the light cycle as physiology is changing, and cells are preparing for the night........

Ben is right, you have to think about which kind of fluo quenching it is you want to relax.  If you just need to relax photochemical quenching (i.e. oxidize QA) seconds will do. If you have substantial NPQ (i.e. if your cells were exposed to high light before sampling) you'll need much longer. Also, 'dark' might not do to realx NPQ (as it requires energy) so very low light is better. Also, because complete darkness get you chlororespiration, which can again quench you fluorescence. Just to be complicated.

Very low far red light is the best option, if you ask me, and the time depends on the light history of your cells.

I have been using 30 minutes that has worked well for my field experiments. Alternatively you can work with them at dawn that may be better to avoid too much light before sampling .