It depends really on the stage of the fermentation, temperature, and batch size. For a typical 20 Liters batch (5 gallons) fermenting at 20 degrees:
It normally takes 24-48 hours for the yeast to start bubbling. This is because during the boiling of beer all the dissolved air is expelled. So a lot of CO2 generated by the yeast will be dissolved in the wort before the airlock starts bubbling. Lower temperature increases the CO2 solubility and decreases the yeast action. Hence lagers take even longer to start bubbling. For the first 3 days (for ales and wine) the fermentation is at its peak. The bubbling rate will be high. About 5-10 bubbles a minute. Once ~30% of the fermentable sugar is converted, the bubbling rates start to fall down drastically. Usually, by the end of the week, the fermentation rate drops to less than a bubble every minute. In the secondary fermentation or aging the airlock rarely bubbles. Bubbling is a sign of either there is a drastic temperature change or refermentation.