doesn't take long at all IF the conditions are right. (light, substrate, co2, filtration). also, a balanced aquascape will help fill the spaces in your tank well.
-bowfront, fluval 209, coralife 65watts 6500k on 12 hours ,1:1 flourite and pea gravel, DIY CO2, fully submerged 120Watt heater temp set at 79 F
-red ludwiga, riccia, microsword, amazon sword, indian water star, lloydiella green, water wisteria
-siamese algae eater, 2 panda cory, 10 neon tetra, 3 algae eating shrimp, 2 rainbow.. err.. rainbow somethings he he