Calculating the exposure time for an object depends on the object's: brightness (magnitude), seeing, sky transparency, sky brightness, moon phase, telescope+instrument+detector sensitivity, wavelength (filter) and the desired signal to noise ("contrast").
In practical terms, this means, for example, that for an object with given brightness, you would have to expose longer during full moon (brighter sky) than during new moon (fainter sky), longer for a blue filter than a red filter (due to CCDs greater sensitivity in the red), longer when the seeing is bad (large e.g. 3 arcsec) and shorter when the seeing is good (small e.g. 1 arcsec), longer for higher signal to noise.
A good, though technical, guide to calculating exposure time is found in the NOAO Direct Imaging Manual for Kitt Peak , written by P. Massey et al.
You can also base your exposure time from the best images of other objects you have observed. Just scale the exposure times accordingly, rememebering that a difference of 1 mag is a factor of 2 in brightness, and that a difference of 5 mag is a factor of 100. Since CCDs are linear devices, simply mulitply your original exposure time by the appropriate factor .
We shall be adding the information for the HOU telescopes as they are available.