I don't think OK has a time limit but it should be done as soon as possible.
The OK probate rules are at http://law.justia.com/codes/oklahoma/2006/os58.html and none of those set a specific time. Many states don't have a time limit and those that do it is generally two years at the very least.
As to the other questions any "interested party" can petition to admit the will. That would include any heir
or potential heir, which includes the grandchildren.
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