Background A societal perspective in economic evaluation necessitates that all resources associated with a disease or intervention should be valued; however, informal care time costs are rarely considered. Objective We estimated time allocated to care by informal carers of colorectal cancer survivors; and investigated the impact of applying alternative valuation methods to this time. Methods Colorectal cancer cases (ICD10 C18-C20) diagnosed 6–30 months previously and identified from the National Cancer Registry Ireland were invited to provide details of informal carers. Carers completed a postal questionnaire. Time estimates per week associated with hospital-related and domestic-related care activities were collected for two phases: diagnosis and initial treatment (initial 3 months) and ongoing care (previous 30 days). Seven valuation scenarios, based on variants of the opportunity cost approach (OCA), and the proxy good approach (PGA), were considered. The base-case was OCA with all carer time valued at the average national wage. Results We received 154 completed questionnaires (response rate = 68 %). Average weekly time allocated to caring was 42.5 h in the diagnosis and initial treatment phase and 16.9 h in the ongoing care phase. Under the base-case, average weekly time costs were €295 (95 % CI 255–344) for hospital-related activities and €630 (95 % CI 543–739) for domestic-related activities in the diagnosis and initial treatment phase and €359 (95 % CI 293–434) in the ongoing care phase. PGA estimates were 23 % below the base-case. Only one alternative scenario (occupation and gender-specific wages for carers in paid work and replacement wages for non-working carers) surpassed base-case costs, and the difference was modest. Conclusions Overall, significant time is associated with informal caring in colorectal cancer. Different time valuation methods can produce quite different cost estimates. A standardised methodology for estimating informal care costs would facilitate better integration of these into economic evaluations.