Tantra; should i be aiming to maintain erection at will and indefinitely or does tantra work if one goes "limp" from time and time? Any advice on this would be appreciated
You can go limp and still maintain sexual excitement and energy. And your erection will come back at some point.
Experiment and see what causes the sexual energy to rise in your body. Deep breathing will help and can slow you down when getting close to the edge.
For me, after getting close and "in the zone", some very quick strokes and then stopping, and doing this several times, generates a lot of sexual energy without going over the edge. Doing Mulabandha during your practice can also make the energy rise. As well as deep breathing.
It gets better with time and practice. And strengthen your PC muscles. This is vey important.
It is not necessary to maintain erection to obtain the benefits of tantric sex. Focus on breathing and energy is more important. You can reach a very deep level of meditation together while being completely still and flaccid as long as you remain connected. My husband and I practice a similar technique to what Dennis described above in the first phase, and then we often rest in meditation for a long time in the second phase.