section 4-4-10/adherence to clearance USA AIM
When ATC has not used the term "AT PILOT'S DISCRETION" nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC. If it is necessary to level off at an intermediate altitude during climb or descent, advise ATC, except when leveling off at 10,000 feet MSL on descent, or 2,500 feet above airport elevation (prior to entering a Class C or Class D surface area), when required for speed reduction.
REFERENCE-
14 CFR Section 91.117.
Mind you, this is for any climb and descent and not for holding patterns alone...so what that means is this: unless you are told differently, the above is FOR ALL ALTITUDE CHANGES including a holding pattern.
It use to be that we climbed/descended as fast as we could until the last 1000' and then went to 500fpm but that changed quite awhile ago.