When should I apply for the METV for Thailand to ensure I don't lose validity?

One final time:

The validity of a visa starts on the date they ISSUE it at the consulate.

Meaning if you get it way before your trip here you burn the validity of the visa down. It's not that important on single entry tourist visas because yoo can only get into Thailand on it once (and you have 90 days from the date it's issued to get here).

On the 6 month tourist visa it is better to get it just before you come here so you're not burning the validity of the visa down when you're not here.

The validity date starts when the passport has the sticker put in it.

Mail to a U.S. Consulate and turn around time should be about 2 weeks (people post this about LA)

The lost time should be about 10 days to two weeks .

The Consulate could confirm on estimated mail time.

If you are in the US you want to apply early enough to allow for mail-time. It's not that big of deal. The 'start date' is about 3 months from when you apply.

The METV has a "valid from" (start) and "valid to" (end) dates. Its a six-month visa. So mine is valid from until . You must enter between these two dates. I think the start date is same date of issue. My ticket into Thailand was - so the start date was date issued.

You must enter Thailand before the end date. If your end date is /2018, and you enter /2018 - you may stay until /2018 (+/- for day months- too lazy to figure it out). This 3 months would be the typical 2 months plus an additional 30 day extension.

Each time you enter Thailand your passport is stamped with with a 'permmited to stay until' date. This date changes - but as a US passport holder on METV - it will be 60 days from the time you enter. You must extend the visa/stamp or leave within that 60 days. The day you arrive and the day you leave are each counted.

I hope this helps.

I suggest you telephone the Honorary Consul in Portland, Oregon. Many times the honorary consulate can be very helpful.