What is the typical lead time for supplier change notifications?  We have 90 days for process/design changes and 180 days for EOL in our contracts.  We are in medical devices and find that the majority of our suppliers struggle with this.

For change notifications, we target 6 months on process/design changes due to regulation approvals and 12 months on end of life due to extensive validation if sourced with on single/sole sourced suppliers.
As you indicated, even this timing sometimes comes up short.

Unfortunately, our company does not deal in device purchases similar to this. My thought is that 90 / 180 would be sufficient, but this seems to be an aggressive timeline depending the requirements of the change. The supplier also has timelines on their side that they may not have control over. i.e. other suppliers, mis-aligned contract terms / SLA's with those suppliers that may cause delays, cash flow to make changes, etc.

EOL is also tricky depending on if it is a unique device only made for you...then you may get into inventory concerns

We have 180 days for change in our standard contracts – negotiation basis, as 90 days is industry standard in electronics. 90 days is typically short as direct materials need to go thru customer approval and may be objected to reliability tests.
Given above, you may look at the nature of the material eg: a process chemistry can easily be changed, while a direct material will struggle on implementation timing and approval, maybe you can try to extend your change period for related materials.
EOL need to be in-line with the related customer project.