In spoken French, the contraction that you can sometimes hear could be written like this:

Quand j’pense

But this is not really a rule, since you do not usually write like that. It is the same for every verb:

Quand j’dis, Quand j’fais, Quand j’mange

and it does not depend on the adverb either:

Ce que j’dis, Où j’vais

The je is always “attached” to the verb and the vowel is removed for the contraction. I would never write Quandj pense or Quand’j pense or anything like this. Also when I speak, I always think of it as attached to the verb.

Also if you stop your sentence just before the verb, you can sometimes hear that this j’ is attached to the verb and not the preceding syllable. For example:

Mais…(break) j’pense pas

and not

Maisj… pense pas

if you see what I mean.

So phonetically, I think that this one is correct /kɑ̃ .ʒpɑ̃s/.

There is a very general phenomenon in French: we don’t like to end a syllable with a consonant. In normal speech (even formal speech), when a word ends with a consonant and the next word starts with a vowel, the consonant from the first word moves to the second word.

car il aime aller à la pêche à la baleine
[ka.ʁi.lɛ.ma.le.a.la.pɛ.ʃa.la.ba.lɛn]

In this example, the r in car and the ch in pêche are attached to the next vowel. The m in aime may or may not stay with its word depending on whether the speaker makes a short pause after the first verb. This can happen with a double consonant as well.

prendre un verre
[pʁɑ̃.dʁœ̃.vɛʁ]

When a word ends with a consonant and the next word begins with a consonant, this may happen, but only for consonant pairs that are considered pronounceable, and then less systematically.

un net refroidissement
[œ̃.nɛt.ʁə.fʁwa.di.s(ə).mɑ̃] OR [œ̃.nɛ.tʁə.fʁwa.di.s(ə).mɑ̃]

When the succession of consonants is too difficult to pronounce together at the beginning of the next syllable, it is fairly common to insert a mini-vowel just to avoid pronouncing the consonants at the end of the previous syllable: this is a schwa. Pronouncing the schwa is more common when the spelling includes an e muet (a letter e which is not ordinarily pronounced), but there can be a schwa purely for reasons of pronunciation regardless of the spelling. The likelihood of a schwa depends on the speaker (schwas are more common in the south of France than in the north) as well as the circumstances (whether the speaker is speaking fast, is making particular effort to articulate, etc.). A schwa may likewise occur when a word ending with a consonant falls just before a pause.

une porte dérobée
[yn.pɔʁt.de.ʁo.be] OR [y.nə.pɔʁ.tə.de.ʁo.be] (or other variations)

Given all that, when a final e is kept silent in words like je, me, te, de, etc. in everyday speech, the consonant is almost always attached to the next word. If this is too difficult, the e is usually sounded.

quand j’pense que t’es pas venu
[kɑ̃.ʒpɑ̃s.kə.te.pa.vny] OR [kɑ̃.ʒpɑ̃.skə.te.pa.vny] OR [kɑ̃.ʒpɑ̃.sə.kte.pa.vny]

Occasionally, with the habit of eliding the e, we find other tricks to deal with the accummulation of consonants. For example, it is possible (but I think not very common) to pronounce je [əʒ] at the beginning of a sentence (with, for once, a syllable ending with a consonant) when it is followed by an unwieldy sequence of consonants.

j’prend un verre
[ʒpʁɑ̃.œ̃.vɛʁ] OR [əʒ.pʁɑ̃.œ̃.vɛʁ]