Inspect your sowings daily. As soon as seedlings appear, remove the covering. Expose seedlings to light and gradually to full sun. Keep in mind that they are tender; too much sun immediately after germination lets moisture evaporate from leaves more rapidly than tiny roots can replace it to the plant. Seedlings may not survive such exposure.
Most seeds of the same kind germinate about the same time and when a fair number of seedlings appear, the plastic is removed. There will be a few late-comers but these will not be unduly affected by removal of the cover. Do not wait for complete germination or you will have too many leggy seedlings that will not grow into strong plants.
As germination occurs, you will notice that even the best seed sown most carefully produces some seedlings that seem more vigorous than others. Although your inclination may be to nurse the weaklings, it is usually wise to discard them. Thinning out the unfit lets in more light and air to strengthen the others. Try to cull the weaklings without disturbing roots of the strong ones. A good way is to snip or pinch the weak plants out close to the soil. (However, with experience you will discover that in some cases, as with highly developed strains of, say, delphiniums, it is the apparent weaklings that later produce handsome plants of particularly fine color, but that is not your concern now.)