It depends on how long a war lasts.
We can keep a minimum of 5 years worth of spares in reserve. For a jet like LCA Mk2, it could be as much as 250 hours a year. And if we assume we plan to fight a war for a month, assuming an insanely high 10 hours a day for 30 days, the total spares consumption is 300 hours, basically just a year and 3 months of peacetime flying. A better estimate would be half that or even less.
In a long war lasting years, production has to be built up. The military has created a roadmap for 100% indigenisation of spares, including for foreign equipment. They may not succeed across the board, but inventories for foreign sourced parts can still be stored for the long term because they are not easy to make and don't have to be consumed quickly as long as it's stored correctly.
