My preamble bits not enough for locking the timing recovery for a gaussian channel
Timing recovery is implemented as a control loop (measure the timing error and adjust the sample offset) and therefore would have an acquisition time depending on the control loop parameters (loop BW). A reasonable estimate for acquisition time assuming the start of acquisition is in the linear capture range of the loop is the first order 10% to 90% rise time estimate of $t_r = 0.35/BW$ where $t_r$ is the estimated rise time from 10% to 90% and $BW$ is the loop BW in Hz. This is accurate for a first order loop and a reasonable estimate whenever there is a dominant pole as that would set the settling time. 

If the Initial state is not within the linear operating range of the loop or if there is lower SNR, this can take much longer. The best approach is to model this in a time domain simulation and observe the timing error versus time with step response testing to evaluate convergence time/ behavior and validate that the loop BW is as predicted from a more detailed control loop model from which these parameters can be computed and help identify issues such as capture range and non-linear behavior. A simulation can also help identify issues related the SNR as well as noise enhancement in the loop. For this purpose the preamble would be treated as infinitely long until it is certain the loop can work in the desired length in all conditions (will this loop operate with carrier offsets or will that be addressed before timing recovery, etc)