If you lay naked in the sunshine your skin can (depending on colour, sunscreen, uvb content time of day etc) theoretically make up to 20,000iu in a short space of time and if you were very clever at managing alternating sun exposure with time for absorption and your skin was hardened to the sun, make up to 50,000iu on a long sunny summer day. So as it is NATURAL to sometime flood the system with D3 it's perfectly safe to do so.
It's standard practice to suggest 50,000iu once a week for eight weeks to correct deficiency
There have been studies using 500,000iu as a loading dose.
My view is that the body naturally usually errs on the side of making more D3 than is needed for daily use. Under the conditions we evolved our DNA would never have had to deal with the parsimonious amounts in most supplemented foods and vitamin tablets. It's hard to imagine any circumstances where full body skin exposure could provide D3 in units' of less than 1000s/iu and so I don't think it's natural to build up to effective amounts. In fact I think it's safer and more natural to go for large amounts and only when you have evidence that you've attained a level over 55ng should you consider reducing the regular supplement intake.

While of course no one should ever let their skin burn, I think even when you are taking an effective amount of supplement there may be other benefits to getting some of your vitamin D from sunshine so I do both, supplement daily AND sunbathe whenever possible.