A synthetic shaving brush works best when you treat lather as an adjustable mixture of soap and water. There is no single loading time that suits every knot, soap and water supply. Start with a repeatable routine, then make small changes based on what the lather actually does.
Wet the brush and control the starting water
Wet the knot with comfortably warm water and gently shake out excess. A synthetic knot does not need a prolonged soak before use. Start damp rather than dripping; this makes it easier to add water deliberately instead of flooding the soap.
Keep the water comfortable to touch and avoid very hot water. Hold the brush by its handle and let the tips do the work. Pressing until the knot is flattened against the soap or bowl is unnecessary and puts extra strain on the fibres and their attachment.
Load enough soap for the job
With a hard shaving soap, work the damp tips over the surface using light pressure until the brush carries a visible coating of soap. With a cream, start with the amount its maker recommends. A practice lather in your palm or bowl can help you learn a new soap before shaving.
Watch the result instead of counting a fixed number of seconds. If your mixture runs out before you have covered the shaving area, begin with a little more soap next time. A larger knot may change your loading routine, but diameter alone does not determine how much product it needs.
Build in a bowl or on your face
In a bowl, use gentle movements and add a few drops of water at a time. On your face, apply the loaded brush to a wet beard, work lightly and add water through the brush tips as needed. Finish with painting strokes to spread the lather evenly.
Look for a coherent, hydrated lather that spreads easily and stays on the area long enough for your pass. A mound of airy foam is not the same thing as a useful shaving layer. Check the feel and behaviour rather than aiming for a particular photograph.
Troubleshoot one variable at a time
If the lather feels pasty, drags or dries quickly, work in a little more water.
If it is thin, runny and full of large bubbles, add more soap or start with less water next time.
If it disappears before the pass ends, reassess both the soap load and hydration rather than pressing harder with the razor.
If a familiar soap behaves differently while travelling, the water and starting wetness are useful variables to check.
Alpha's Bandit with a 26mm G5 knot and BetaHive with a 28mm G5 knot provide two synthetic examples. Learn the water-and-soap balance of your own brush rather than expecting different knots to load identically.
Rinse right through the knot
After shaving, rinse until no lather remains, including near the base of the fibres. Gently remove excess water and leave the brush somewhere airy to dry. Avoid pulling or twisting the knot and do not seal it in a travel tube while damp. Consistent rinsing is easier than trying to remove accumulated soap later.