How to increase the EFI splash screen delay time on Virtualbox (5.1.18)?

It looks like EFI is still an experimental feature on VirtualBox 5.1.18. Its user manual goes as far as to say. :

Note that the VirtualBox EFI support is experimental and will be enhanced as EFI matures and becomes more widespread. While Mac OS X and Linux guests are known to work fine, Windows guests are currently unable to boot with the VirtualBox EFI implementation.

source: http://download.virtualbox.org/virtualbox/5.1.18/UserManual.pdf (pg. 59 - "Section 3.13 Alternative Firmware (EFI)")

Also, in the same guide it covers all the flags/switches available for the VBoxManage modifyvm command. In that section it provides a way to increase display time for BIOS but it does not for EFI. Your desired functionality does not look to yet exist in VirtualBox.

I would recommend switching back to BIOS and using --bioslogodisplaytime to adjust the BIOS splash screen afterward:

VBoxManage modifyvm "VM name" --firmware bios
VBoxManage modifyvm "VM name" --bioslogodisplaytime 3000