6-8 weeks, yes. It simply takes that long - closer to 8 really - for any parasites to get to a developmental stage where the chemicals will kill them.
I thought I remembered to use a double dose of strongid or safe guard, so is ivermectin the recommendation now?
Yes and yes
There are 2 issues. 1 is that there’s a growing resistance of ascarids to ivermectin. Ascarids are EVIL for foals.
The other issue is the high, widespread resistance of strongyles to Strongid (pyrantel pamoate) and Safeguard/Panacur (fenbendazole).
This means you cannot count on ivermectin to kill ascarids (though in many places it does, I just wouldn’t rely on it, the risk is too high) and you cannot count on the other 2 chemicals to kill strongyles.
Thankfully, double dosing fen and pp do kill ascarids effectively, and thankfully, ivermectin is still effective against strongyles.
So, the best protocol is to alternate, every 4 weeks, between single ivermectin, and either double pp or double fen. The double fenbendazole is a bit more effective against ascarids than the double pp, overall. You can even use oxibendazole if you don’t want to double fen each time, and the oxi is even a bit more effective than the fen against ascarids.
One bonus of double pp is it also kills tapeworms
At the 6 month mark, you can use Equimax in your ivermectin rotation. It’s not totally necessary if you just did, or will do next time, the double pp, but see above about the fen being a bit more effective against ascarids, so I’d use that for your non-ivermectin rotations