Natural gas furnace ignitors are not made to last as long as the furnace so you will be replacing the furnace ignitor before the furnace gives out.
Furnace Ignitors are expected to last from 4 to 7 years before needing replaced.
I keep a new furnace ignitor in my toolbox since a furnace ignitor only costs around $24.00 but to have a furnace repairman replace the furnace ignitor can cost from $75.00 to $200.00 depending on labor cost.