I'm no expert at this but these are some of the things that I would consider thinking about. How practical is it for you to count the total number of bees in the hive, based on lets say hive weight or the number of bees on one rack in the hive (then multiply by the total number of racks to give you an estimate of the total bee numbers in a hive) before you start collecting video data? This will at least put your influx/outflow number into some sort of prospective if you are counting in different times of days or if there are different wheather conditions in different days. This is easily achieved if you divide your influx and outflow numbers by the total (estimated) number of bees. This gets you two numbers (ratios) which you plot on the Y-axis. On the x-axis you can plot the passage of time and even colour code different days based on average (lets say) temperature to see if temperature had an obvious effects on your observations. But obviously all your instances of recording and time intervals between recordings needs to be constant!
So in many fly behavioural or locomotion function assays, scientists record fly movements at certain fps and analyse the videos. Now most of this video data analysis is done using scripting in Matlab which is (I know to be) the tool choice for motion analysis. Now since you say you are at high school, this would probably be quite a challenge since the program is costly and scripting is not that easy (at least for me), so see if you can utilise the help of someone to get over this barrier. Once you do then you can analyse much longer videos at much higher frequencies, which will give you a better resolution on your data. If you could't then consider looking through web sites using search engines such as google for Matlab codes for motion tracking analysis scripts. I did this some time ago and there are wealth of information out there and once you do that you can get the communities help in websites such as stack overflow to help you refine your codes or any problems you may have with it, so long as you show evidence of your work. This is a great option since you can add variety of data (or different data aspects) to your project such as what is the average speed/velocity which your bees travel at in different times when entering/exiting the hive, is there a pattern to their travelling (e.g. straight line, zig-zag etc) and much more. I'm aware that this is beginning to sound like a PhD project and not a high school project but these are some things to consider. If programming is totally out of the question then you can look on internet to see what free programs are out there to aid you with your data collection. I briefly looked and saw this (http://www.youtube.com/watch?v=jWD8CHW06Ks) and this (http://www.researchgate.net/post/Does_anyone_know_of_FREE_software_for_motion_capture) just by simply typing into google "video motion tracking software" so look around and see if you can find anything which you can tailor for your own need.
Now regarding video length time, as correctly mentioned in Armatus response (if I'm not wrong and correct me if I am) it shouldn't matter for how long or how frequent you are recording so long as you keep the variables (i.e. time of recording, frequency of recording etc) to the minimum (i.e. not change them), or in other words keep your observation methodology constant through time, and try to come up with a practical schedule and if you explain your thought rational (experimental design) in your report, even a professor (i dare to say) wouldn't be able to argue with it but you always have to talk about the limitations of your study and that is key and that's where peoples project falls apart when an expert reviews it and finds holes and criticises claims made, which can not be backed up by the data/types of analysis gathered/performed!!
EDIT: Just to address some statistics issues, I would make your recordings from minimum of three hives per recording time-interval/point since in science n (number of observations) should be >= 3.
Anyways I hope my ramblings have been somewhat useful.