Viburnum

Rhododendrons blooms for 2-3 weeks the most. Hydrangeas on another side may bloom for about 5-6+ weeks.
Clethra is easy growing shrub that bloom for 3-4 weeks. There is a wide range of cultivars that bloom from June to September.

Kerria japonica is supposed to bloom as well in shade as it does in sun. Rainbow leucothoe does not have the bloom time but it's multicolored foliage makes up for it.

Here is a link that might be useful: rainbow leucothoe

My hellebores are still blooming since february.

Endless Summer hydrangeas

Here is a link that might be useful: Endless summer hydrangea