A common approach is to take your shortest recent cycle length and subtract 17. On a 28-day cycle that means starting around cycle day 11; on a 32-day cycle, around day 15. Cycle day 1 is the first day of full flow.
If your cycles vary by more than a few days, start earlier rather than later and test once daily, moving to twice daily as the line begins to darken. The surge can be short — in some people under 12 hours — which is why once-daily testing occasionally misses the peak.
Kits with a generous strip count exist precisely because a full picture usually takes more tests than people expect in the first few cycles, while you learn your own pattern.