Marinate for as long as you can. Two hours works, overnight is better, and 24 hours is genuinely the best of the lot. Do not go past 24 hours, as the lemon starts to break the texture down.
Freeze it raw and marinated, not cooked. Bag the marinated thighs flat and freeze for up to 3 months. The marinade keeps working as it thaws, so it often comes out better seasoned than a fresh batch.
Chicken is done at 74°C (165°F) in the thickest part. Thighs are forgiving, so a few degrees over will not dry them out.
No oven or grill? A cast iron pan gives the best char of the lot, 6 to 8 minutes a side in batches. An air fryer at 200°C for 12 to 14 minutes, in a single layer, is the least smoky option.
It will smoke at 250°C. Use the extractor and open a window. That fierce heat is what chars the outside before the inside dries, so do not be tempted to turn it down.