Infrared thermography is a promising non-contact inspection method for detecting subsurface defects in concrete structures. The detectability of defects using solar radiation is primarily influenced by factors such as defect geometry, specimen orientation, wind speed, and other environmental conditions. Past research has predominantly focused on environmental factors with limited emphasis on the influence of defect dimensions and depth. This study presents an experimental investigation on reinforced concrete slab specimens with simulated defects of varying sizes (300 mm, 100 mm, and 50 mm) and depths (25 mm, 50 mm, 75 mm, and 100 mm), covering a wide range of size to depth (s/d) ratios from 0.5 to 12. The slab specimens were monitored continuously for 24 h to examine defect contrast evolution during heating and cooling cycles. Under the tested environmental conditions, defects with an s/d ratio of 2 or greater were detectable. The common period during which most defects were detectable was found to be in the early afternoon. These findings can be used to determine the optimal time for conducting field surveys of bridge decks.