This link on the ARRL website breaks it down clearly. Don’t let your renewal go too long — if it’s been expired for more than two years, it’s not eligible for renewal or reinstatement.
http://www.arrl.org/call-sign-renewals-or-changes
“Amateur applications can be filed manually using paper forms or electronically via the FCC website. Filing interactively is easy and the results are immediate.
To renew or update a license, amateurs may file manually using FCC Form 605 or file interactively on the FCC ULS website using their FCC-issued Federal Registration Number (FRN) and password. FCC permits administrative updates and call sign changes at any time. License renewals are allowed at 90 days or less before a license will expire, and when the license has expired but is still within the two-year grace period for renewal. Licenses that have been expired for more than two years are not eligible for renewal or reinstatement.”