Honestly, at this point, war footing for Europe or America would at best cut the time in half. And that is being generous. Truthfully it may shave months off at best due to the lack of meaningful factories to convert.

On the other hand, we have a lot more diverse arms beyond artillery shells that would both need production increases as well as stockpiles we would empty in the meantime. Aerial bombs, other ground based missile platforms, possibly even other artillery shells that are simply not compatible with Ukrainian systems currently in use.

America in particular is not a heavy user of artillery for the prior 50 years, and mostly as defensive weapons currently. And the American military would be pushing to turn it from a defensive to offensive war as soon as possible.