When there is enough food to keep them around. That would be late spring or early summer.

I wouldn't bother after the experience I had with them this summer. I released ladybugs for an aphid problem I had following directions on the label, and the next day, very few of them were anywhere to be found. I will never spend money on ladybugs again.
Mrs H

Yup. That's what happens.

As it happens, ladybugs are genetically programmned to fly, fly away home once they wake up for their winter dormancy.

And since the ladybugs sold to gardeners are collected while dormant, they "wakie up" in your yard and then they leave!

When choosing a ladybug for use as a general predator in your pest management program, you need to ask your supplier many questions. One question you should be asking is "where do these ladybugs come from?" Some suppliers sell ladybird beetles that have been "harvested" from natural winter aggregation sites. This means ladybugs in the western United States after feeding head up into the mountains by the millions to spend the "off season" in huge hibernating masses. These are usually convergent lady beetles, Hippodamia convergens. Collectors come along and scoop up the hibernating ladybugs and shipped them out for release. Once released into a nursery or landscape they usually migrate before feeding or laying eggs, providing little or no control for your target pest. Another concern is harvested ladybugs may be parasitized by a small wasp, Perilitus coccinellae. It develops as an internal parasite of lady beetles and kills them. Harvesting from the wild is also not environmentally sustainable.

It better to plant plants that will attract the native ones in.

!!
(::)
Buglady

I live in Middlesex County Virginia. Come to my home and you can HAVE all of the lady bugs that you desire! Yuck, you're welcome to them. I just vacuumed a bunch of them for the second time this morning...and it is only 10:15 a.m.