  The full disk, full of gas and dust, determines the upper limit of planet
masses, and its lifetime is critical for planet formation, especially for giant
planets. In this work, we studied the evolutionary timescales of the full disks
of T Tauri stars (TTSs) and their relations to accretion. Combined with Gaia
EDR3, 2MASS, and WISE data, 1077 disk-bearing TTS candidates were found in
LAMOST DR8, and stellar parameters were obtained. Among them, 783 are newly
classified by spectra as classical T Tauri stars (CTTSs; 169) or weak-lined T
Tauri stars (WTTSs). Based on EW and FWHM of Ha, 157 TTSs in accretion were
identified, with ~ 82% also having full disks. For TTSs with M<0.35 Mo, about
80% seem to already lose their full disks at ~ 0.1 Myr, which may explain their
lower mass, while the remaining 20% with full disks evolve at similar rates of
non-full disks within 5 Myr, possibly suffice to form giant planets. The
fraction of accreting TTSs to disk-bearing TTSs is stable at ~10% and can last
$\sim$ 5-10 Myr, suggesting that full disks and accretion evolve with similar
rates as non-full disks. For TTSs with M>0.35Mo, almost all full disks can
survive more than 0.1 Myr, most for 1 Myr and some even for 20 Myr, which
implies planets are more likely to be formed in their disks than those of
M<0.35 Mo, and thus M dwarfs with M>0.35Mo can have more planets. The fraction
of full-disk TTSs to disk-bearing TTSs decreases with age following the
relation $f\propto t^{-0.35}$, and similar relations existed in the fraction of
accreting TTSs and the fraction of full-disk CTTSs, suggesting faster full
disks and accretion evolution than non-full disks. For full disk stars, the
ratio of accretion of lower-mass stars is systematically lower than that of
higher-mass stars, confirming the dependence of accretion on stellar mass.
