That depends on a lot of things, but certainly splitting too soon is a bad idea. Personally, if you have some control over the timing (like they're not already making swarm cells or you don't have some kind of deadline to meet) I'd shoot for two weeks before the main flow and just take the old queen and all the open brood (except one frame) for the split. This maximized the foraging force in the old hive because they have no brood to care for, it happens at a time of year when the weather is nice so you don't get chilled brood from not enough bees to keep the brood warm, it gives the bees a chance to make a new queen at a time of year when there are lots of drones and lots of food, and it helps with Varroa because the old hive skips one whole brood cycle where they would have been raising Varroa and dividing their workforce raising brood DURING the flow that won't be available DURING the flow.http://www.bushfarms.com/beessplits.htm
But, I certainly wouldn't split a hive that had less than bees on ten frames. More would be better.
Bees need a certain critical mass before they can really take off. That means there have to be some "spare" bees to take up the slack, raise more brood, forage, keep the brood warm on cold nights etc. When there is a surplus of bees the hive can really take off. When there is not they often struggle.