Translation:It takes ten months.
The じっ pronunciation is the older form. At some point, people started using じゅっ under the influence of じゅう, basically replacing the /i/ vowel with /u/. Originally, 十 was pronounced zipu. By itself, it became zifu → ziu → zyū → jū, a series of regular sound changes (it was still spelled じふ in the pre-1946 orthography). However, when combined with counters such as ka, a different set of changes occurred, zipuka → zipka → zikka → jikka
It is. That's another way of pronouncing this kanji combination. In most counting in Japanese, whenじゅう is shortened to じゅっ it can also be shortened to じっ instead. Probably as a non-native Japanese speaker it's best to stick to じゅっ when saying it yourself, but be able to recognize what a native speaker is saying when they say じっ.