There's some sort of circle gauge on the buddy pokemon that falls as time runs after the feed but I'm not sure when it becomes feedable. I'm guessing there's some indication but it's not clear enough.
Depends on what you mean by "feedable". There are two main things:
- Feeding the buddy to fill the gauge and get hearts:
- The indicator you mentioned is divided into 6 sectors and is fully filled when you feed your Pokemon 3 regular berries (Pinab, Nanab, or Razzberry) or 1 Golden Razzberry. At this point you buddy will receive one heart and joins you on the adventure, so you can earn hearts for walking, battling, interacting with new pokestops. This state will last for 3 hours, so each sector will represent 30 minutes. However, you will be able to refill the gauge only after 2 sectors were completely depleted so the earliest would 1 hour after initial feeding. The indicator will change appearance to reflect sectors in the buddy play mode at this time. If you do it right on the hour, you might need only 1 berry to refill the gauge, but the timing is tricky, so you might need 2-3 regular berries or 1 Golden Razzberry. Once you refill the indicator, your buddy will gain another heart (up to 3 on regular state) and it will take another hour to repeat the process.
- Using Poffin fills up the gauge and all the 6 hearts (3 doubled) and will last for 6 hours. Each sector on the gauge represents an hour, but feeding your buddy will not have any affect until those 6 hours elapsed.
- Feeding your buddy to get him superexcited without the Poffin. This can be done every 30 minutes, but will not count before 30 minutes has passed since the previous activity, so the easiest way to manage it would be setting a timer for 30 minutes right after feeding a berry. After several iterations the hearts for the main activities are doubled and you can get more hearts by filling the gauge fully again. There are separate tutorials on activities and timing to get this done.