DD 1 (5 and in reception) does 2 hobbies..
Two nights a week she does gymnastics after school. One night we have friends over for playing, one night they just chill at my mum's house and one night we go to soft play / park in summer.
On Saturdays she has ballet, tap and modern for 1 1/2 hours but that doesn't start till 11, so we still have an easy morning.
Sundays, we sometimes go to kids cinema club together, otherwise we just chill.
Homework is always done at the weekend, so we have no worries there and I do not find her getting tired or stressed at all, if she did we wouldn't do it.
DD2 (2) has two hobbies - ballet (half an hour) and gymnastics (3/4 hour) two afternoons a week.
Its not stressful, or hard work and in fact is rather chilled. Both DDs thrive on their hobbies, and have learnt balance and skills etc. DD2 (2.3) is really good at skipping , jumping, roly polys, walking on tip toes and so on...) I think they get loads out of it, and given that schoolwork is sooooo easy, its not a stress at all. I think it is nice to see a child achieve and learn, and when DD1 gets a new badge, or learns a new skill, she is positively brimming with excitement. Plenty of years for worrying about homework and academics imho - I think the early school years should be about having fun, enjoying themselves, learning new skills and gaining confidence... hobbies can help that (has really helped my two) and so in my opinion is worth its weight in gold.
But that's just my view!