Brick sessions, most commonly bike/run, form the cornerstone of the training week during the build and peak phases in a training plan. I’d suggest that one per week is adequate, but they shouldn’t always be the same distances or on the same routes. You should vary the length of each distance so that you focus on specifics, for example a hilly bike or a flat multi-loop run. The key advice is that you should aim to run off the bike at the kind of pace you hope to run during the complete run leg of a triathlon, which helps to create good muscle memory and improves your discipline to hold consistent pace rather than flagging off towards the end.
For moving up to middle-distance try a 70km bike followed by a 15km run. This allows you to get close to race distance, which is great for finding your race pace and forces you to be disciplined. Try to finish the session feeling strong and knowing you could run the extra 6km at the same pace. This distance also allows you to fine-tune your fuelling strategies.