Can anyone advise how long a US C-1 transit visa is valid from the date of issuance? I assume it is multi-entry since I will need to transit in both directions. I just need to know how far in advance we can apply for the visa. Thanks!
The answer depends in part on your nationality. You can look up the default validity period at https://travel.state.gov/content/travel/en/us-visas/Visa-Reciprocity-and-Civil-Documents-by-Country.html/.
The consular officer may however issue the visa for a shorter period, but this is uncommon as far as I know.
Since the visa may be valid for up to 10 years (for some nationalities), and the fee for B and C visas is the same, you might consider applying for a B-2 visa instead.