US and Ukrainian officials have said that Russia could declare formal war on Ukraine as soon as 9 May 2022. (CNN report). A formal declaration of war is necessary under Russian law for a general mobilization of Russian society under a wartime footing, which would allow Russia's military to draft conscripts and reserves en masse, in order to solve its current manpower shortages.
According to Michael Kofman, senior fellow at the U.S. Center for Naval Analysis (CNA), "Without a national mobilization, the Donbas is the last major offensive the Russian military can attempt given the current state and availability of forces." (Source) (See also "War on the Rocks" podcast discussion here.)