I went back to work when DS was 3 months old. At that time I had built up a 100oz freezer stash to hold me over if I ever needed to make up a shortage. I ended up just rotating through that so that nothing was ever frozen longer than 3 months total
For the first few months I pumped three times a day at work. Luckily I worked only 4 days a week, and one of those was from home, so I only actually had to go into the office on M/W/F. I still sent DS to daycare for a portion of the day I worked from home, but that meant less bottles for that day.
I had a hard time finding enough time at work to keep up with the 3 times a day schedule, so I added another pump session in every night (even on the weekends, yuck..)after DS was in bed to make up the difference. I pumped 2 times a day at work from the time DS was about 6 months until he was a year, and dropped down to one time a day and slowly weaned by pump at about 14 months.
I continued to send bm to daycare with DS until I used up my stash and at around 15 months he was totally drinking cows milk when away from me but still nursing 2-4 times a day until we weaned right around 18 months.
Good luck! I can't say that pumping is fun, but it did give me a huge satisfaction to know that I was able to exclusively bf my son his entire first year and beyond and still work nearly fulltime.