Hello there:
Normally if your ignitor is coming on in the specified time usually around 45 to 75 seconds
this would indicate that the thermostat is defective and needs to be replaced.
However if the ignitor its taking longer that this this would indicate that the ignitor is getting weak and should be replaced.
I would recommend replacing the ignitor along with the thermostate to ensure that you are using new parts you will get better performance with new parts ok
Hope this is very helpful for you
Best regards Michael