Depending upon where you were in the world.......
Canadian AIM RAC 8-5-1 Paragraph b)
If the phrase “when ready” is used in conjunction with an altitude clearance, the change of altitude may be initiated whenever the pilot wishes. When the change is initiated, the pilot should advise ATC. Pilots may temporarily level off at any intermediate altitude; however, pilots are expected to advise ATC of any temporary level-off at any intermediate altitude. Compliance with assigned or published altitude crossing restrictions and speeds is mandatory (CAR 602.31—Compliance with Air Traffic Control Instructions and Clearances), unless specifically cancelled by ATC. (MEAs are not considered restrictions; however, pilots are expected to remain at or above MEAs)