くらい

After a number, くらい means "about": じゅっぷん10分くらい, "about ten minutes". It goes straight after the number and counter. ぐらい is the same word, and either is correct anywhere.

With a time of day, くらい follows the time and any particle comes after it: さんじ3時くらいにきてください来てください. For a point in time, ごろ is the more usual word.

"So … that"

After a verb or an adjective in the plain form, くらい gives the degree of something by saying what it led to: "so tired I couldn't walk".

"As … as"

After a noun, くらい compares: "as tall as", "about the same as".

"At least", "just"

くらい also plays something down as small or easy: "at least", "something as simple as".

くらい or ほど

ほど also gives an amount or a degree, and sounds more formal. For "not as … as", ほど is the standard word, and the one to learn first.