R3 controls the flash duration. It needs to be kept small in relation to R1. At 10K the voltage on the capacitor barely needs to increase at all to trigger the thyristor. To slow the flash rate, increase R1 or the capacitor.

R3 controls the flash duration. It needs to be kept small in relation to R1. At 10K the voltage on the capacitor barely needs to increase at all to trigger the thyristor (since the resistor R3 is in series and the current through R1 also passes through R3). 

To slow the flash rate, increase R1 or the capacitor.