Upon entering a thermal, the standard rule of thumb says postpone that initial turn three or four seconds. This brings you further into the rising air rather than immediately turning away, while also allowing time to determine if the lift is really worth stopping for. Most inexperienced pilots tend to turn for lift too soon even if they do go the correct direction, inadvertently swinging back and away from newfound lift. It’s generally best to wait until the lift peaks before turning, and, depending on a thermal’s size, this waiting period could last ten seconds or more.
When you’re cruising into the wind, thermals will be rising toward you from ahead on course, making it possible to delay that initial turn indefinitely. If you were to turn too soon you’d immediately below the rising air, and need to move further upwind to rejoin the thermal at a lower altitude. By delaying the turn, just slowing up on course may bring you to the top before you reach the thermal’s far (windward) side. If you wait too long and fly beyond the lift, a quick reversal will put you right back in it.
When running downwind, it pays to turn sooner. Otherwise, if you’re moving fast or the tailwind is strong, you might shoot through lift and beyond it very quickly. If you find that you have turned too soon, you’ll know the lift is right there immediately downwind, while more is rising up from directly below you.