If you want to get reasonable life from any type of incandescent lamp when flashing, you need to have a "keep warm" circuit.
This will actually pass a small current through the lamp, when it is off. Not enough to produce a visible light, but enough to keep the filiament hot. This reduces the thermal shock, and lets the lamp come up to full bright faster.
I do this at work with PWM, using a reduced rate during the off cycle, and it vastly extends the life of the lamps. One thing to note about halogen lamps.. the halogen design produces a very high level of heat inside the lamp to attain its design effect, which is to help recombine some of the filiament boiled off during use. Lowering filiament temperatures can prevent this. Therefore there is a trade-off when using halogen lamps.. to slow a duty cycle can shorten their life.