Some people argue that contemporary science can't be right about how old the earth or the universe is, because an omnipotent being wouldn't need to take that long to make a universe. Thus the young earth must be true. Others argue from the opposite end that intelligent design arguments are inconsistent with an omnipotent being, because they involve God inputting information over a long process. (See, for example, SteveF's July 25 comment at 5:47 pm in this comment thread on this post.) I don't think this sort of argument works in either case.
If the designer is God, then God should be able to do something over a longer time or a shorter time. Young earth creationists are right that God could have created everything instantly. But the argument undermines the young earth view as much as any other, unless the young earth view holds God to have created everything instantly. It doesn't hold that but takes the period of creation to be six days. Why would God need six whole days to create everything? God could have created instantly. If it's implausible for God to do something over thousands or millions of years (because God could do it in a shorter time), then it's implausible for God to do it in six days (because God could do it in a shorter time). The mere possibility that God could have done it over a shorter time does not mean that God would have done so. A divine being with omnipotence could choose to work over a very long time or a very short time, and neither should seem more or less likely without an understanding of the purposes such a being might have for working over a longer or shorter time.
The hypothesis that there is a designer, particularly if one of the possibilities is that the designer is omnipotent, does not make it more or less likely that the designer worked over a long or short time. The length of time is not evidence against God. What's interesting is that the reverse is not true. Length of time issues may count as evidence against naturalistic explanations, precisely because they do not involve beings who can do anything (and thus can work instantly or over a longer time).