Re-entry permit (I327) takes about 4 months to process (NSC). You don't have to wait in US until it is approved. You can leave US when it is pending.
You can apply for re-entry permit as soon as you are LPR and you must file it when you are physically in US. Generally, there is no limit of how many times you can apply for re-entry permit. However, if you stay outside US for more than four years of last five years, your re-entry permit will be valid for only one year unless you are on religious missionery or work for US government overseas.
You can apply for re-entry permit as soon as you are LPR and you must file it when you are physically in US. Generally, there is no limit of how many times you can apply for re-entry permit. However, if you stay outside US for more than four years of last five years, your re-entry permit will be valid for only one year unless you are on religious missionery or work for US government overseas.