ずっと details

the whole time + write

zutto kaku

the whole time; all along — one unbroken stretch, and in a comparison, by far

meaning: write the whole time

返事へんじが来くるまで、ここでずっと待つまつつもりだ。

henji ga kuru made, koko de zutto matsu tsumori da.

I intend to wait the whole time right here until a reply comes. neutral

新あたらしいほうがずっと使つかいやすいよ。

atarashii hou ga zutto tsukaiyasui yo.

The new one is far easier to use. casual

Examples

the whole time; continuously — one span with no break in it

返事へんじが来くるまで、ここでずっと待まつつもりだ。

henji ga kuru made, koko de zutto matsu tsumori da.

I intend to wait the whole time right here until a reply comes. neutral

子こどもの頃ころから、ずっとこの街まちに住すんでる。

kodomo no koro kara, zutto kono machi ni sunderu.

I've lived in this town ever since I was a kid. casual

事故じこの後あと、電話でんわがずっとつながらない状態じょうたいが続つづいております。

jiko no ato, denwa ga zutto tsunagaranai joutai ga tsuzuite orimasu.

Since the accident the phones have been unreachable throughout. formal

by far — the gap in a comparison is much wider than expected

新あたらしいほうがずっと使つかいやすいよ。

atarashii hou ga zutto tsukaiyasui yo.

The new one is far easier to use. casual

今年ことしの冬ふゆは去年きょねんよりずっと寒さむいです。

kotoshi no fuyu wa kyonen yori zutto samui desu.

This winter is much colder than last year's. neutral

続ける
〜続つづける is a compound verb and needs a verb to attach to, so no adjective or noun predicate can host it; ずっと measures the span from outside the predicate and takes states as readily as acts — ずっと元気げんきだ works where the compound has nothing to hang on.
いつも
いつも counts REPEATED occasions and ずっと measures ONE unbroken stretch, so the two split a sentence's meaning rather than its register — いつも待まっている says the waiting happens on every occasion, ずっと待まっている that a single wait has never let up.
まだ
Both report a state still in force, but ずっと measures the span behind it and まだ only says the change has not come — ずっと待まっている calls the waiting long, まだ待まっている merely says it is not over.