bloom from late spring till frost.  Most of the plants will have one major flush of flowers and then bloom sporadically.
Most varieties will benefit from being divided every 3-5 years and I tend to shear mine back to the central growth point in the spring and fall to keep a bushy look.

G. endressii 'Wargrave Pink' - The most commonly grown geranium. Salmon pink flowers. 18-24" tall. Zones 3-8

'Rozanne' - A violet blue hybrid that's 2008 Perennial Plant of the Year. 18-24" tall. Zones 5-8.

'Ann Folkard' - One of the earliest blooming geraniums, with magenta flowers that repeat bloom throughout the season. Trailing habit. 6-8" tall. Zones 5-9.

'Double Jewel' - Double-white petals with a lilac center. It’s short and perfect for containers. 10" tall. Zones 4-8

G. oxonianum 'Southcombe Double' - Double, pure pink blooms that resemble fluffy asters. 10" tall. Zones 4

'New Hampshire Purple' produces magenta-purple flowers on compact mounds of green deeply notched leaves that turn a crimson-red in the fall. 'New Hampshire Purple' will adjust easily to both hot summer and cold winter climates. This cranesbill is one of the most vigorous hardy geraniums and is ideal for the front of a sunny border, although it will cope with a fairly shady spot. Cranesbill is generally quite adaptable to a variety of growing conditions. Though it prefers moist, well-drained soil, it will also tolerate periods of drought once established. For this reason, it is a good choice for southern gardens. It is best to lift and divide large colonies in spring. Their carefree growing habits make them an ideal low maintenance plant that will grow anywhere and provide years of beauty.