How far in advance can I apply for a B2 visa?

Assuming you actually need a B1/B2 Visa that requires an appointment, the Visitor Visa site has the following to say about when to apply for a Visa

Wait times for interview appointments vary by location, season, and visa category, so you should apply for your visa early. Review the interview wait time for the location where you will apply:

And includes a handy calculator where you can enter the U.S. Embassy or Consulate where you will be scheduling your appointment and see what the current wait time is.

For the sake for some real numbers, I entered "London", and it says as of today that the wait time is 200 days for a visitor visa. So applying at least 7 to 8 months ahead would be the bare minimum amount of time that you should think about. And to answer your question, 12 months wouldn't be unreasonable.

Note that Visa Appointment Wait Times page has the same calculator, but has some additional explanation about the process. One of the things that this page mentions is the possibility of expedited processing of your visa. Here it notes (my emphasis)

Note: Travel for the purpose of attending weddings and graduation ceremonies, assisting pregnant relatives, participating in an annual business/academic/professional conference, or enjoying last-minute tourism does not qualify for expedited appointments. For such travel, please schedule a regular visa appointment well in advance.

Before you apply far in advance, you should check the visa reciprocity schedule for your country of nationality. As you can see from the notes for the United Kingdom, it is your country of nationality that determines the maximum validity of the visa, not the fact that you are applying with a UK-issued refugee travel document:

Refugee Travel Documents: The new Refugee Travel Documents issued by the United Kingdom are machine-readable. The Travel Documents include the Certificate of Identity which is brown in color, a travel document for refugees under the 1951 Convention which is dark blue in color, and a travel document for refugees under the 1954 Convention which is red in color. All documents contain a disclaimer stating their issuance is "without prejudice to and in no way affects the holder's nationality." All list either the country of nationality or of birth, or both. The nationality of the bearer is listed as "Unknown". The documents will also indicate whether the bearer has "indefinite leave to remain" or the "right of abode."

Nonimmigrant visas issued to bearers of these travel documents should reflect the visa reciprocity schedule of his/her nationality or origin, even if the bearer has "indefinite leave to remain" in the United Kingdom.

Old style Refugee Travel Documents issued by the United Kingdom are still valid until they expire. These include a light brown travel document and a light blue Certificate of Identity. These documents also indicate whether the bearer has indefinite leave to remain or the right of abode in the United Kingdom.

If you are stateless, your visa will be issued according to the Temporary Reciprocity Schedule, which means that it will be valid for three months. (This means that you will be able to use the visa to enter the US within three months of its date of issuance. It does not mean that you will have to leave the US before the visa expires.)

Also note that the fee cited in the reciprocity table is an additional "reciprocity fee" on top of the usual $160 nonimmigrant visa application fee. Therefore, the entry "Fee: none" does not mean that the visa is entirely free of charge.