Estimating the telescope time needed for a scientific programme is obviously the task and responsibility of the investigators. There are no absolute rules for such an estimation, as it depends, besides the expected signal strength and S/N ratio, on the line width, the requested baseline quality, the pointing and calibration accuracies, etc... Experience, however, shows that many observers grossly underestimate even the time needed for standard quality observations during good weather. Since time underestimates may result in 6-12 month delays in the derivation of usable results (or, worse, in the publication of inconclusive results), as well as in an increased bureaucracy, everyone's interest is to prevent them as much as possible. The following gives some simple examples and formulas aimed at helping you to make realistic time estimates. They are in no way, however, rules, and should not curtail your own personal experience. Note that IRAM and the IRAM Programme Committee realize fully that a (slight) overestimation of the time needed to carry out a programme is preferable to the reverse. The rating of programmes is done accordingly.