Do you intend to initially purchase an one-way (OW) and then change it to a roundtrip (RT)?
Probably a bad idea.
If not a North American trip, for most destinations, the OW will be more expensive than the RT. Plus when you book the RT, the entire travel including the outbound will be repriced. Not real advantage to booking early.

If in North America , just book as two OWs

If you plan to book the RT and shift the return after traveling, then return will be repriced based on the new date. The problem is the return can not be extended beyond 12 months of the original purchase date. (new tickets are only good for 12 months, while UA is allowing 24 months for pre-1 April tickets). So you probably can not book until within 12 months of the return. You might get agent t go longer but that is not a certainity.