Hallo Werner,

unfortunately, I didn't find reliability data of T89C51CC01.
But, under the assumption that FLASH memory technology and fabrication is very similar for all ATMEL's '51-derivatives, I would take reliability data of other ATMEL FLASH micros and make a conservative estimation.
Reliability data is available for AT89C51 (4KB), AT89C52 (8KB) and AT89C55 (20KB). Because micros with bigger FLASH memory are containing much more memory cells, it's obvious, that data retention time for these is expected to be accordingly smaller.
'Retention bake test' at 150°C, which simulates an highly accelerated stress for devices, yielded the following total device hours (t) without any failure:

AT89C51: 3,342,000 hours
AT89C52: 2,852,000 hours
AT89C55:   480,000 hours

According theory gives an Arrhenius' acceleration factor of:

Af = exp [Ea / k x (1 / Tu - 1 / Tt)] = exp [0.5 / 8.6171D-5 x (1 / (40 + 273.16) - 1 / (150 + 273.16)] = 124

Ea = 0.5eV is the activation energy of failure mechanism, k = 8.6171D-5 is Boltzmann's Constant, Tu is use condition junction temperature and Tt is test condition junction temperature.

The failures in time (FITs) are estimated with the help of chi-square distribution:

lambda = chi-square / 2 / Af / t x 1D+9 = 1.833 / 2 / 124 / t x 1D+9 = 7,391,129 / t

chi-square = 1.833 represents 'zero failures' and '60% confidence level'.

Inserting the different total device hours (t), we get the following failure rates:

AT89C51:  2.2FIT, which corresponds to a MTTF (mean time to failure) of 57,078years,  
AT89C52:  2.6FIT, means MTTF of 43,906years,
AT89C55: 15.4FIT, means MTTF of 7,413years.

Can you live with MTTF of about 7400years?

Keep in mind, that using different confidence level will have some influence on failure rate. When chosing 99% confidence level, MTTF is 5 times smaller!

Hope it helps,
Kai