goes to look for numbers

https://isis-online.org/isis-reports/detail/alabugas-greatly-expanded-production-rate-of-shahed-136-drones/

This is projecting maybe 6,000 Shaheds produced at Russia's Alabuga facility by August 2025.

Alabuga documents show that the Shahed drone production facility was designed to make 10 drones per shift (see Figures 4 and 5). Assuming a five-day work week and 52-week operation, the plant operating at one shift per day could produce about an average of 217 drones per month or about 2600 drones per year. The contract quantity for 2023 was smaller, at an average of 6.9 drones per day, but the actual production rate for 2023 was on average 11.8 drones per workday. This means that Alabuga was operating slightly more than one shift per day and significantly exceeding its planned production rates for 2023. For 2024, Alabuga planned in its contract with Iran for an average monthly production rate of 226 per month, or about 10.4 per workday, about one full shift per workday. However, daily average production appears to have increased again in 2024, for an average of 18.8 drones per workday, or 407 drones per month, or two shifts per workday. Table 2 shows these results.

https://en.wikipedia.org/wiki/MIM-23_Hawk

Approximately 40,000 of the missiles were produced.

We've never fired the HAWK in war ourselves, and I've read that relatively-few have been used. So in an optimal scenario that said missiles haven't been scrapped, all countries holding them are willing to provide MIM-23 missiles, that missiles haven't decayed, that multiple launches are not done against individual targets to improve kill chances, and that no MIM-23s have been destroyed on the ground, at present production rates, total historical MIM-23 production would be able to last something like eight years of Shahed strikes. But my guess is that those are probably optimistic assumptions.

I also don't know whether Russia may have access to sources of Shahed drones other than from Alabuga, like new Iranian-made drones.

The MIM-23 missiles that it uses are an old missile, have not been produced for a long time.

Solid rocket motors (which power these missiles in flight) have a limited shelf life. I couldn't find numbers specifically on the MIM-23 shelf life, but other solid rocket motors range from 10 to 50 years. With the age of the HAWK systems, its entirely possible that these missiles are close to or beyond their "Best use by" date where you wouldn't trust them as primary air defense. So using them against Shahed is a fine choice. You're essentially using throwaway munitions of your own against throwaway munitions of your adversary.

Possibly expired HAWK missiles against Shahed is a much better choice than Patriot against Shahed.

This is already the transition, from nothing to something.