Looking at my flight ticket's arrival and departure dates, it shows a duration of stay of 45 days. Can I still apply to STV visa (I plan to cancel the return trip and book a ticket for a later date), or must it be a regular tourist visa because it is less than 60 days?

The return ticket should not be a problem when applying for a STV.

I don't recall their being a return ticket being required when applying for a STV.

FYI I called the LA embassy: they told me day 45 days must be regular tourist visa and can't be STV.

32 minutes ago, user70 said:

FYI I called the LA embassy: they told me day 45 days must be regular tourist visa and can't be STV.

The embassy in Washington DC states this "2.4 Proof of flight confirmation/reservation, indicating the trip from the US to Thailand" but I suspect the e visa site will ask for a departure ticket.

Soruce: https://thaiembdc.org/2020/12/28/special-tourist-visa-stv/  (LA consulate website takes you to the same page for the STV See: https://thaiconsulatela.org/en/visa/visa-type/)

Either change your reservation or get  a temporary onward ticket (search for owner ticket) to submit for the application.

LA is a official Thai consulate. There can only be one embassy in a country.

I just flew there in November and my flight back was for 2 months later. I was questioned in Narita airport because I was entering on a tourist visa 30 day. I just told them I’m allowed to extend it for 30 days and showed them I had done it before in my passport and then they said ok. No issues when arrived to Thailand going through immigration at all. 

Got my STV from London last year. One way ticket.

On 2/24/2022 at 7:50 AM, user70 said:

FYI I called the LA embassy: they told me day 45 days must be regular tourist visa and can't be STV.

Considering that an STV is intended for tourists intending to spend 90-270 days in Thailand, requiring an onward ticket out of Thailand is ridiculous, especially in Covid days. Few are going to want to make firm plans for what they will be doing nine months from now, and forcing people to buy throwaway tickets to satisfy the embassy when applying for a visa is nonsensical.