Unfortunately, there isn't a threshold income to meet before you have to file an S-Corp return.
From the IRS Instructions for Form 1120-S:
A corporation or other entity must file Form
1120S if (a) it elected to be an S
corporation by filing Form 2553, (b) the
IRS accepted the election, and (c) the
election remains in effect. After filing Form
2553, you should have received
confirmation that Form 2553 was
accepted. If you didn't receive notification
of acceptance or nonacceptance of the
election within 2 months of filing Form
2553 (5 months if you checked box Q1 to
ask for a letter ruling), take follow-up
action by calling 1-800-829-4933. Don't
file Form 1120S for any tax year before the
year the election takes effect.
