I want to find someone that may have experience with legal requirements for implementing electronic billing in Mexico over SAP ECC 6.0 EhP7. How was the experience, how long did it take?

No Experience in this, apologies

I know some people working in HPE or DXC in Mexico who may fit in your comment or help you, as they have been deploying these type of system for several customers and they know Mexico legal requirements. Anyhow, if this ebilling is with Public Sector customers, this will have some specific requirements to be fulfilled

ALso, depends on the level of eBilling you want to make. Ebilling can be simply a matter of sending a pdf electronically through a general portal, or generating the invoice with .xlm format, getting connected directly with the customer AP system? depending of the level of integration, you will require more or less experience, In general, for instance, for generating the invoices in pdf format, using Pagero Portal, may take 3-6 months depending on the complexity and level of support you may have. This time is from the system specification to final user tests and put in production.