From the Act
Recission
"Upon transmission of such special message, the President may withhold certain funding in the affected accounts for up to 45 legislative session days. If a law approving the rescission is not enacted within the 45 days, any withheld funds must be made available for obligation."
Deferrals
"The ICA defines a “deferral” as withholding, delaying, or – through other Executive action or inaction – effectively precluding funding from being obligated or spent. The ICA prescribes three narrow circumstances in which the President may propose to defer funding for a program: (1) providing for contingencies; (2) achieving budgetary savings made possible through improved operational efficiency; and (3) as specifically provided by law. The ICA requires that the President send a special message to Congress identifying the amount of the proposed deferral; the reasons for it; and the period of the proposed deferral. Upon transmission of such special message, the funds may be deferred without further action by Congress;"
https://budget.house.gov/publications/report/impoundment-control-act-1974-what-it-why-does-it-matter What I am wondering is, what is a legislative day? check out the 2020 calendar. That 45 days could take forever.
https://www.cop.senate.gov/legislative/resources/pdf/2020_calendar.pdf
If it's a deferral, it's almost indefinite. I don't think it can be used to run out the clock until the budget expires.