Hi Kade
Make this timer is easy, but first you need to tell which type of arduino are.you using. After you need to.check ph and EC otherwise your roots can be burn from this parameters. And then pumping timing have to change by external parameters, as light and temperature. Then you need control also humidity for external plant grow
I have a vertical hydroponic system, so I can help you, but not giving you the code, but only how can you do it, otherwise you are not push to learn.
Let me know