i'd get a test.. the serum values would be pretty clear on that one i think. with 5,000 iu of d3 daily, i would aim for 400mg of magnesium from FOOD and another 100-200mg of magnesium from a highly absorbable non-laxative form like magnesium glycinate (VERY absorbable) or magnesium citrate (a bit less soluble, but 200mg shouldn't give you GI troubles)
as for a timeframe, it has taken me years to get to the point where i am not quickly symptomatic without daily magnesium supplementation. I've made a huge effort where diet is concerned. I've been lax lately, did a washout for bloodwork and haven't really taken anything lately. diet has not been gold star either. today i can feel my mag levels are suffering, by the background tension in my forearm muscles. it has taken a few weeks for it to get this bad.
make no mistake it's not an easy thing to deal with, takes sustained long term effort. i'd consider backing off on the daily d3 just to give your body a chance to build up some magnesium stores. and i certainly would never turn my nose up at a serum magnesium test
i just smile and nod when the doc says 'normal, and aim high high high. if i take too much, my muscles will start to feel weak. too little, they feel tensed and tight. bloodwork helps keep it all on track.