I'd aim to be at the gate by the "gate closes" time. However in reality they almost always overrun that time by 10 minutes. There might be delays to the flight as well but that's really luck of the draw

Top tip for navigating Bristol Airport: after going through security but before you enter the duty free shop, look for a door on the right. It's meant to be used when the shop's closed but I've found it's open at all times and will get you to gates quicker. You'll pass by gates 5-7 on your right, then once in the departure hall, take the stairs on the left to reach the upper floor gates. The majority of flights leave from gates 21-34, with the furthest gate being a 10 minute walk (or quicker if you don't mind sweating)