I'd take guess that judging by the 'do... it looks like circa 1965 or 1966 perhaps. That would pre-date Thomas Crown Affair.
For all my other fellow Dusty boffins, turn to page 78 in 'The Complete Dusty Springfield' and it looks like exactly the same wig Dusty is wearing in the photo which was taken 1st Feb 1966 at the Waldorf.
She wore this style of wig for Hullabaloo when she sang 'Some Of Your Lovin' and for the front cover of Lucy O'briens book i think, if my memory serves me correctly. Never thought that particular style really suited her.