When you're expressing at home to build up your supply and a stash of milk, once a day is plenty. The aim is not to drive you crazy! Once you go back to work, standard practice is to express once for each missed feed. So for most moms, this is probably about three expressing sessions in a typical eight-hour work day. It can feel like a lot, especially when you’re trying to get a job done too. As a result, if you really can’t fit them all in then come up with a plan that works for you. For example, express when you get up, before work, after your baby goes to bed or during the night. 

Other articles you might find useful for understanding expressing: 

How much milk should I express?

Can I heat breast milk that has been expressed?