This is a very interesting question. I cannot pinpoint without researching any references on the subject, but I have encountered the situation and have successfully specified lifetime in terms of economic life in the past.

Some of the approaches to specifying lifetime:

• designed mean time to failure (failure is defined as failing to meet specified requirements). This is useful in entrepreneurial product development in unregulated sectors, for non-repairable items, and is therefore in a product development spec in these circumstances). Verification requirement: Verify by analysis based on manufacturer-claimed and/or historical reliability data for components, and company historical data for connections).
• designed mean time to critical failure (critical failure is defined as failing to meet specified requirements classified as critical to fitness for intended use). This is useful in entrepreneurial product development in unregulated sectors, for non-repairable items, and is therefore in a product development spec in these circumstances). Verification requirement: Verify by analysis based on manufacturer-claimed and/or historical reliability data for components, and company historical data for connections).
• mean time to failure (failure is defined as failing to meet specified requirements). This is useful in entrepreneurial product development in unregulated sectors, for non-repairable items, and is therefore in a product development spec in these circumstances). Verification requirement: Verify by thorough reliability testing to a confidence level of not less than xx%.
• mean time to critical failure (failure is defined as failing to meet specified requirements classified as critical to fitness for intended use . This is useful in entrepreneurial product development in unregulated sectors and regulated sectors, for non-repairable items, and is therefore in a product development spec in these circumstances). Verification requirement: Verify by thorough reliability testing to a confidence level of not less than xx%.
• design economic life (economic life is defined by some objective economic criterion, such as $Net Present Value per year to sustain compliance with requirements (or a defined subset of requirements). The formula for estimating sustainment cost is a part of the requirement, and usually involves an established life cycle cost estimating algorithm.) Verification requirement may be to verify through independently performed analysis.
• economic life (economic life is defined by some objective economic criterion, such as $Net Present Value per year to sustain compliance with requirements (or a defined subset of requirements). The formula for reducing sustainment cost and its individual ingredients to reference year NPV is a part of the requirement.) Verification requirement may be to verify through independently performed analysis, or verify historically through measurement (or both).
• economic life (economic life is defined as the period of time for which the value achieved (or achievable) from use of the item exceed the loss of value through cost of sustainment, each per unit of time). There are variations on this theme, where other cost elements such as opportunity costs, financing costs, disposal costs, etc. are factored into the calculation. This approach is good for large infrastructure projects such as buildings, stadiums, roads, etc. Verification requirement may be to verify through independently performed analysis, or verify historically through measurement (or both).