Dear Sourish,

In case of mice, there is only one 6 minutes trial in which the first 2 minutes is not considered for evaluation and the next 4 minutes is considered for behaviors such as immobility, climbing and swimming. 

Another important thing is that you need to do some experiments (both FST and Open-field test, n= 6-8) to assess the average immobility time of your C57BL6 mice. To do this, you need to consider some factor such as the SEX of animals and  AGE of animals. If you are doing FST on female mice, please note that you need to report it in both estrus cycles of animals because the effects of hormone on the immobility time. Please note that adolescent mice have different behavior in comparison with adult animals. Also, there are some other factors that are important such as time of the test. For example, it is important that you are doing the test in the dark cycle or in the light cycle of these animals. 

And finally, there is no normal or accepted immobility time among different mice strains or even for C57BL6 and you need to optimize the test in your LAB. There are some papers that may help. Good Luck

1- Forced swimming test in mice: a review of antidepressant activity. 

DOI: 10.1007/s00213-004-2048-7

2- Assessing antidepressant activity in rodents: recent developments and future needs

DOI: 10.1016/S0165-6147(02)02017-5

Dear

The forced swimming test needs a training period before the records of the control and the tested groups of adult mice. As the original method stated to do this training period. Then you must report the immobility period with proper matching of variations among the control group involving the inter and the intra-individual variations by good stat. analysis to match a well-selected matching mice in the control group from hich you can report the range of the immobility time to the test group (s). This should be done by " blinded" investigtors meaning that they don't know except that they must report the immobility time by a stopwatch or a camera inserted on the bath according to the lab. conditions. 

Please review the following references:

Petit-Demouliere, B; Chenu, F; Bourin, M (January 2005). "Forced swimming test in mice: a review of antidepressant activity.". Psychopharmacology. 177 (3): 245–55. doi:10.1007/s00213-004-2048-7. PMID 15609067.

Borsini, Franco, Giovanna Volterra, and Alberto Meli. "Does the behavioral "despair" test measure "despair"?." Physiology & behavior 38.3 (1986): 385-386.

Jump up ^ Porsolt, RD; Le Pichon, M; Jalfre, M (21 April 1977). "Depression: a new animal model sensitive to antidepressant treatments.". Nature. 266 (5604): 730–2. doi:10.1038/266730a0. PMID 559941.

 Porsolt, RD; Bertin, A; Jalfre, M (October 1977). "Behavioral despair in mice: a primary screening test for antidepressants.". Archives Internationales de Pharmacodynamie et de Thérapie. 229 (2): 327–36. PMID 596982.

 Can, A.; et, al. (2012). "The Mouse Forced Swim Test". Journal of Visualized Experiments. 59 (3638): e3638. doi:10.3791/3638. PMID 22314943.

 Detke, MJ; Rickels, M; Lucki, I (September 1995). "Active behaviors in the rat forced swimming test differentially produced by serotonergic and noradrenergic antidepressants.". Psychopharmacology. 121 (1): 66–72. doi:10.1007/bf02245592. PMID 8539342.

 "Porsolt Forced Swim Test — Penn State University". Research.psu.edu. 2013-04-29. Retrieved 2014-03-24.

 Borsini, F; Meli, A (1988). "Is the forced swimming test a suitable model for revealing antidepressant activity?". Psychopharmacology. 94 (2): 147–60. doi:10.1007/bf00176837. PMID 3127840.

 O'Neill, KA; Valentino, D (12 March 1982). "Escapability and generalization: effect on 'behavioral despair'.". European Journal of Pharmacology. 78 (3): 379–80. doi:10.1016/0014-2999(82)90043-7. PMID 7067732.

 "Mice and Rats in Laboratories". PETA. Retrieved 2016-11-02.

Thanks a lot, Shayan, Sahar and Laura. I did do the 6 min test with 2 min pretest/habituation and 4 minutes analysis.