I rehomed a Spaniel 2 years ago, she's working cocker or wcs x springer, she's quite big even for a wcs and does have the 'boing' of a springer, but I know cockers do too, so who knows.
I think they will keep going as long as they think you want to and they never appear to tire, so mine is just as engaged at the start of a walk as at the end and will keep chasing the ball at full speed as many times as I'll throw it. She's also a velcro dog, so if I'm moving so is she, I think it's the inbuilt cooperation that they always think you have a job for them when you move or are just interested & curious in what you're doing.
I think people see them like that and assume there's no off switch, but she'll happily have a duvet day and not mither for a walk or play and be content with belly rubs and cuddles all day.
She lived outside and had the run of a farm with the other dogs before she came to us, and I used to go daily to the farm and most of the time they were found snoozing in the straw when I arrived, they would come up the fields and run constantly the entire time, but rarely would they be up there if no one was there with them, they'd all be chilling around the yard, playing or just mooching around quietly. So left to their own devices they weren't constantly on the go, they had every opportunity to be and weren't.
I don't think being on the go constantly just because they will is probably the best thing for them, and I think people mistake that they will for thinking they must, and a lot are over stimulated and don't get enough rest.
She's seriously the best dog I've ever had, so trainable, affectionate and eager to please. I have a jrt who I love to bits but the difference in trainability and motivation is obvious, the spaniel wants to do whatever she thinks I want her to do because she just wants to please me, everything is a transactional trade off with the jrt! She wants to please herself!