Update the actual figures at least monthly, and revise the assumptions whenever something material changes — a large contract won or lost, a customer starting to pay late, a price increase from a key supplier. A forecast that is never revised stops being a forecast and becomes a record of what you once hoped would happen.
Many businesses run a rolling short-term forecast covering the coming weeks alongside a longer view covering the year. The short-term version drives day-to-day decisions; the longer one supports planning, tax provisioning and funding conversations.
The most valuable part of the review is comparing forecast against actual. Persistent differences tell you something about your assumptions — usually that customers pay more slowly than your terms suggest, or that costs are lumpier than you allowed for.
If you are approaching a lender, an up-to-date forecast with realistic assumptions strengthens your case. Optimistic figures that are quickly contradicted by your bank statements do the opposite.