Magnetisation transfer in a TOCSY experiment occurs via a different mechanism than for COSY. The spinlock generates an oscillatory exchange of coherence between spins that is propagated thoughout the coupling network. The efficiency of the spinlock transfer depends on a number of parameters; molecular conformation and topology which effects the coupling constants, and relaxation rates across the spin network being the most important. So, yes, signal does depend on the size of the coupling, but occurs via a different mechanism.
Once the spins are brought into the transverse plane through a standard excitation 90 and allowed to evolve during t1 period (as per a COSY), they are 'locked' into position using low cw power, or usually by a composite pulse train. Those signals that are in-phase following the evolution period are aligned with, and locked to, the transverse axis, effectively removing the influence of the primary magnetic field, Bo. The spin frequencies are now proportional to this new spinlock field, which is very small compared to Bo (so the chemical shifts are effectively all the same, because chemical shifts/transition energies are proportional to the strength of magnetic field), and so all of the spins are strongly coupled and this allows efficient mixing of magnetisation fur the duration of the mixing period. This is called isotropic mixing.
Typical values for spinlocks range from about 30ms-120ms. There is no hard and fast rule for predicting the optimum mixing time. Magnetisation transfer ocurs faster between spins with large coupling constants, so only 2,3J couplings will be seen with short mixing times. There are various rules of thumb used for selecting the appropriate mixing time. You can use 1/(10JHH) where JHH is the coupling across these transfer steps. A typical JHH is about 7Hz, so to see a 5 step transfer you might choose a mixing time of:
5 x 1/(10x7) ~ 70ms
Another guestimate is that around 15ms will give you results similar to a COSY, and you can add 15ms for each transfer step, which will give you something like:
- A->B 15ms
- A->B->C 30ms
- A->B->C->D 45ms
- A->B->C->D->E 60ms
- A->B->C->D->E-> 75ms
I would usually suggest values of 40-70ms as a good starting point. For larger molecules, you will lose signal through relaxation with longer mixing times. Also, longer mixing times give wider distribution of magnetisation, which means an overall reduction in total signal-to-noise
It is not uncommon for research samples to record 2 TOCSY experiments using different mixing times; one short and one longer.
A TOCSY is not necessarily a solution to a system that contains small or vanishing couplings (not weak, as you suggest). The TOCSY experiment still requires coupling to exist along the spin system, and it is the break in the spin-coupled pathway that halts the distribution of magnetisation that allows individual spin systems to be identified.