You can extend it up to just over four seconds by messing with this:
Paste this in a text file, then change the file's extension to .asm, and you can then apply it to your ROM with Asar.
40 is the default value for this setting. Change it to however long you want the intro to last, in frames (1/60ths of a second); the number is in hexadecimal, so the maximum is FF (255 in decimal).
If you need it to last longer, a longer code is required, but hopefully this should be enough for your needs.
Professional frame-by-frame time wizard. YouTube
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