Out of the 8 companies I've worked at as a product engineer, the most successful framework to deliver tangible results has been Basecamp's Shape Up approach. Engineering managers always ask "how many days" of effort when the real question they should be asking is "how much appetite"/"how long do we want to spend" on the particular product/feature we want to build? The Shape Up framework was the only time I didn't feel constantly stressed and it actually provided time to cooldown between the six week cycles. And the fact is it actually led to very successful product deliveries consistently. For those interested here's a link to it https://basecamp.com/shapeup/0.3-chapter-01.