Bulking means deliberately eating more calories than you burn for a period of time, so your body has the energy and building material for new muscle. The opposite phase is the cut, where you lose fat in a deficit.
In bodybuilding the two phases often alternate. The bulk is when you gain weight and accept some fat along the way. How much fat depends mostly on the rate: a small surplus is a lean bulk, a large unplanned one a dirty bulk.
How fast a bulk should go
A review on off-season nutrition for bodybuilders (Iraki et al. 2019) suggests about 0.25 to 0.5 percent of body weight per week for beginners and intermediates and around 0.25 percent for advanced lifters. In your first year of training you can lean toward the upper end, after several years toward the lower end.
Example: At 80 kg, 0.25 to 0.5 percent per week is 0.2 to 0.4 kg, or just under 1 to 1.7 kg a month. This is a worked example, not a target for you.
Measure this rate with your trend weight, not with single daily readings, because the scale can swing by a kilogram or more from day to day. In the first few days it often rises faster, as glycogen and water fill back up.
When the bulk ends
There is no fixed rule. Many people end the bulk when body fat has climbed to the point where they no longer feel comfortable or the following cut would get too long. A planned end date or target weight keeps the bulk from turning into slow, open-ended fat gain. Waist size, photos and a body fat estimate help with the decision.