It depends on the exact flasher you use. Most (almost all) have an "alternate flash rate" either faster or slower, that's supposed to alert you to a burnt out signal bulb. Some don't have this "feature" but I don't know of any way to tell before you try one. The advantage of this flasher from superbrightleds is that it doesn't have that, it flashes at around 90/min regardless of load.
I use a Tridon 3-wire flasher that flashes fast with LEDs but this is OK with me and I could modify it to flash slower if I really wanted to, it just needs a resistor cut and a jumper installed on the circuit board. Normal two wire flashers have problems: the electromechanical ones will not flash with LEDs and the electronic ones will flash LEDs but they flash "off first" - when you switch on the signal the lamp is dark for a half second or so then starts flashing. That bothers me, I want the lamp ON immediately when I turn the switch. The three wire electronic ones start with the lamp on. Three terminal electromechanical flasher don't work with LEDs either.