I just finished an away so here are my thoughts:
-The ideal months to do an away rotation are June or July. The reason so is because it's early enough for you to get an LOR in time for the SFMatch. The main drawback is that these months are hard to secure for an away because many students at their respective home institutions will do their home ophtho rotation at this time as well. If you do it in July, you'll be able to catch the new PGY2s coming onto ophthalmology.
-August and September are a bit later but doable as well. I did mine in August at a very strong program. The advantages of doing a rotation these months is that you'll be more prepared considering you did a previous ophtho rotation at home. In addition, you also have the recency effect, in that you'll be remembered much easier during interview season if you worked a lot with the residents and attendings. The disadvantage is that you'll likely be not able to get a LOR for the SFMatch because you'll be cutting it too close to the target date. However, you can secure a 4th LOR as an accessory LOR and send it directly to programs.
-October is almost not worth it. The reason I say that is because if you get an interview in late October it may look bad at the program you're rotating at. Yes, people will easily recognize you during interview season, but if it's for the bad reasons, it will hurt you. At this point you're better off knocking a mandatory elective like ambulatory medicine.
As far as the number of away rotations, if you worked your @ss off during your home month rotation and during one away rotation, that's probably all you need to do. Several residents I've talked to from two different institutions have told me they've done two rotations max, some with only one done at their home institution. With your third and fourth ophtho rotations, you may love the field, but the cost and fatigue of doing away rotations may catch up with you. If your reasoning to do more than 2 ophtho rotations is to increase your chances or rotate at places that you really really want to match, then by all means go for it. My experience has been that after two rotations you will probably know more than is expected for an M4 clinically and technically and you'll be ready to start as an intern.
Last if you're a student with a very strong ophthalmology program, the consensus seems to be to NOT do an away rotation. The reasons being is that poor performance may hurt you more and letters from strong attendings at your institution should be enough (ophtho is truly a smaller world than you think).
Just my 2 cents. Take with a grain of salt and your experiences may differ.