I have leftover udon broth, can I use other pasta (ei. spaghetti)? And, how long does the broth last?

Spaghetti is a fantastic idea to be able to use up your left over broth. I usually like to look in my pantry before having to buy something. Soba and or sweet potato noodles are great substitutes.

I dont know how much you have but i usually freeze it in individual portions in a ziploc freezer bag.

The broth freezes well.

Yes...one can use leftover broth with other noodles. The taste will be good but different.
Don't let broth sit at room temp over two hours. Keeps in the fridge about 3-4 days after cooking.