I take an evening to sit down and configure/learn the controls on the camera. With a new lens I'll take some 'real world' test shots in and around the house but nothing involving test charts or the like. Then I'll usually take it out on a foto-shoot the next weekend, always nice to use new gear
Any fault should quickly become apparent if you start using the stuff before the return period is over.
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 2.0x III, 70-300L | E-mount: SY12 f/2, 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses