How often should budgets be reviewed and revised?

based on the type of budget. for example, sales budget is reviewed more often than the capital budget.

also the change in circumstances

Normally the budget estimates are reviewd priodically with the management accounts as the comparisons give an accurate bench mark for revision. An extra review my be needed if there is a change in the environmental factors.

Persoally I reviewed budgets monthly looking for variances, (+/-) seeking answers particularly for overages.  Adjustments are made caustiously.  Depending upon the type of accounting used, monies need to be transfered from somewhere (another fund or line item) so adjustments I made were only considered quarterly.