There is no one definitive answer to this question as it depends on a variety of individual factors. However, in general, 30 minutes of moderate-intensity stationary biking can be an effective way to promote weight loss.
For starters, 30 minutes is a significant chunk of time to dedicate to exercise, and research has shown that longer bouts of exercise are more beneficial for weight loss than shorter ones. In addition, stationary biking at a moderate intensity can help you burn a decent number of calories. For example, a 155-pound person will burn approximately 423 calories in 30 minutes of moderate-intensity stationary biking (at a rate of about 14 calories per minute).
Of course, the number of calories you ultimately burn through exercise depends on how much you weigh - the heavier you are, the more calories you'll burn. But even if you don't lose any weight immediately from your 30 minutes on the bike, regular aerobic exercise like this can have other benefits like improving your cardiovascular health and helping to reduce your risk for obesity and chronic diseases down the road.
So while there's no guarantee that 30 minutes on the stationary bike will lead to immediate weight loss results, it is certainly worth incorporating into your routine if you're looking to slim down over time.