即时 is common in tech contexts: 即时通讯 (instant messaging), 即时翻译 (real-time translation). For everyday 'immediately,' 马上 (mǎshàng) or 立刻 (lìkè) are more natural. Compare 及时 (jíshí, timely/in time) — different character, different meaning.
mistakes
Don't confuse 即时 (jíshí, instant/real-time) with 及时 (jíshí, timely/in time). They sound identical but 即 means 'immediate' while 及 means 'to reach/in time for.'
