I have had two dogs throughout graduate school. I rescued both of them prior to returning to school, so they were already used to my crazy work schedule (used to work FT at a hospital, then PT at a bar/restaurant). It's definitely challenging at times, but for me, so rewarding!
I probably work 50-60 hour weeks. Mine are both crate trained and are in their crates maybe 8-10 (sometimes 12) hours/day, depending on my schedule. Often times, I come home and they're both passed out, snuggled in their blankets, and don't realize I'm there. Since I am gone so long, I try to make the time we do have together worthwhile -- walks, snuggles, belly rubs, etc. Weekends I try to do longer walks/runs and work at home, if possible.
I tend to adjust my social schedule around them. For example, if the plan is happy hour, I aim for 5 pm drinks so I can be home by 7 pm. If the social activities start at 7 pm, I try to leave school around 4/4:30 pm, so I can feed them and walk them before I leave again.
I was able to drive them back to my parent's house for the holidays this year and my parents watched them while I did 3 weeks of internship travel. That helped out a lot! I have a lab mate who watches them when I go away for short trips and I board them at the vet when I go away for longer trips and/or my lab mate can't watch them. I know things like that are not always possible, but finding a person who loves animals but can't currently have them or trading off dog-sitting with another dog owner can help you out immensely when it comes to money!
Overall, my dogs have been great stress relievers and I don't think they've altered my graduate school experience.
The only downside is the cost. Having two means double the food costs, double the vet bills, double the boarding costs, etc. And you have to plan for the unexpected. One of my dogs scratched his cornea last year, which required surgery and several vet visits before it healed. That was ~$1000 I wasn't planning to spend, but **** happens! I do not have health insurance for my dogs, but others might -- that's another $25+/month and something to consider.
Also, I agree with above posters that you should really take the time to investigate dog breeds. While all dogs are adorable, their energy levels can really affect your overall well-being. Personally, I love boxers.....I think they're the cutest dogs; however, I will never own a boxer b/c I do not have the same energy level. Running with my dogs on a Sat/Sun works well for me. Running with my dogs every day, maybe twice a day, does not. I personally chose smaller dogs b/c of apt living and b/c they're a little spazzy sometimes, but are mostly lazy -- either laying with me on the couch or laying outside in the sun.
Good luck with whatever you decide to do! You could always volunteer at your local shelter walking the dogs to get your dog fix if you're not yet ready to adopt/rescue. 🙂