It depends on the compound or compounds being used, whether they're orals/fast-acting injectibles or longer-acting injectible esters.
With most orals you can begin to notice something usually within a week or so, major results 2-3 weeks in. As far as longer-acting injectible esters you'll usually notice something three or four weeks in, with significant gains materializing somewhere around week six or so. This is a broad generalization and will vary from person to person and depend on exactly what compounds are being utilized.