Every International conference I have seen announced typically makes the announcement a year to 6 months before the conference. Remember that many attendees will not only have to put together a paper for submission, but also work on funding their trip, acquiring requisite visas etc. In addition, think about the strain on your paper reviewers. The review process takes time. I would think that any where from 6 months to three months would be fair.
An early announcement of a conference (say, a year or so before) would be good. (You can use "Conference Alert" site for that). This early announcement would alert potential presenters about your event and give them sufficient time to prepare a paper, etc. (As stated in the post above, sometimes the trip plans and visa need to be arranged in advance too).
As to the opening and closing dates, it would be good to open for paper submission 4-6 months before. The closing date could be practical to set at 1 months (at any rate not later than 2 weeks) before the event begins. It is critical not to underestimate the importance of a proper closing date. This is because the organizer needs sufficient time to prepare the conference program, booklet, etc. If papers keep coming 1-2 weeks before the conference it could be difficult and strenuous for the organizing committee to finalize the program and make other numerous arrangements.
Good luck with your conference!
It depends on couple of things. From my experience most important are two.
First: traffic origin of your participants and periodicity of conference
- participants mostly known researchers (usual case for a project related conferences or ongoing cooperation) - in such case you might not need a wide gap as orientation time frame will be commonly known to the parties. Periodicity is not relevant.
- participants mostly unknown researchers. Already established conference also does not need a wide gap, but there usually already is orientation from previous periods.
- participants mostly unknown researchers, this is first conference expected to turn in to annual or so. You need a wide gap but no longer that a year as the day of conference should not be before the conference day even if year differs. You might loose possible participants because of slight overlook.
Second: Available human resources
- if you have many willing reviewers you can shorten the term
- if you have few willing reviewers and they might need each to review few papers or you think you might need to find some extra reviewers, then have this in mind while deciding on last submission day: will there be enough time for reviews before the conference?
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