Formal — the standard line on customer-service phone systems, hotel front desks, and bank counters. In casual speech use 等一下 or 稍等; 请稍候 stays on the polite/formal end.
mistakes
Don't confuse 候 ('wait') with 后 ('after'). 请稍候 = 'please wait briefly'; 稍后 (shāohòu) = 'a little later, in a moment'. Different characters, different grammar — 候 is a verb, 后 a time noun.
