Megi
At our school, they don't expect YR to sit for periods at a time. However they do "label" up the tables so that when they come to do "table-based" activities, then they will be placed with children of similar abilities.
I am a volunteer helper in both Reception and Year 1/2 classes. My main job is to take children from Year 1/2 classes out of the classroom to an area where they have "free-play", including things like role-play, junk-modelling, construction, arts and crafts. They do this EVERY morning for an hour.
In Reception, I supervise their outdoor play in the morning. At our school, they play outside for 45 minutes every morning - riding bikes, playing in the playhouse, water/sand play etc. They are free to wander from outside to indoors and to go into the other Reception class.
Reception is very much "free-play" - so far this week, my DD has been playing in the "home corner", lots of painting and water play.
They probably sit for a maximum of 10 minutes at a time doing a "table-based" activity - but from observing I do think "streaming" according to abilities does work - there is no point providing an activity to a group on the Number 10 when some of them are still struggling with Number 2.