In parshat chukat it says that if you become tamei due to a dead body, you become pure again by being sprinkled with red-heifer ash-water on the third and seventh day of your impurity, but if you don't then you remain impure... 12 the same shall purify himself therewith on the third day and on the seventh day, and he shall be clean; but if he purify not himself the third day and the seventh day, he shall not be clean. יג כָּל-הַנֹּגֵעַ בְּמֵת בְּנֶפֶשׁ הָאָדָם אֲשֶׁר-יָמוּת וְלֹא יִתְחַטָּא, אֶת-מִשְׁכַּן יְהוָה טִמֵּא--וְנִכְרְתָה הַנֶּפֶשׁ הַהִוא, מִיִּשְׂרָאֵל: כִּי מֵי נִדָּה לֹא-זֹרַק עָלָיו, טָמֵא יִהְיֶה--עוֹד, טֻמְאָתוֹ בוֹ. 13 Whosoever toucheth the dead, even the body of any man that is dead, and purifieth not himself--he hath defiled the tabernacle of the LORD--that soul shall be cut off from Israel; because the water of sprinkling was not dashed against him, he shall be unclean; his uncleanness is yet upon him.
So what happens if you miss days 3 and 7 of the sprinkling (either intentionally or on purpose). Are you tamei forever?