Plutarch gives Caesar's debts as thirteen hundred talents:

λέγεται πρὶν εἰς ἀρχήν τινα καθίστασθαι χιλίων καὶ τριακοσίων γενέσθαι χρεωφειλέτης ταλάντων

It is said that before he obtained any magistracy he was a debtor of one thousand and three hundred talents.

That's the only actual figure we're given, but Caesar's debts only rose as his political career went on--at least, that is, until his consulship in 59 and certainly by his proconsulship he was financially secure. Thirteen hundred talents of debt before even embarking on a political career is a chunk of change, and just about any Roman politician would run up debts canvassing for office--taking out loans from your buddies was a major method of political alliance that the senatorial class employed. Plutarch remarks right after that passage that Caesar spent enormous sums of his own money while curator of the Via Appia, and again during the games he presented during his aedileship, which featured some 320 pairs of gladiators along with banquets, processions, and lavish theatrical performances. Plutarch remarks that the images of Marius which he had made displayed publicly (for the first time in decades) were gilded and inscribed and generally awesome, and expensive--these were, like the other works of his aedileship, presumably paid for out of pocket. Catulus offered to bribe Caesar to prevent him from running for pontifex maximus, but Caesar refused and took out even more money in loans, though we don't know how much. Plutarch claims that Caesar's campaign while propraetor in Spain left him a wealthy man, absolving him of his debts, though--Suetonius claims that while in Spain he embezzled money from his allies and sacked Lusitanian towns that had already surrendered to him to pay for his debts (although both of these are common slanders against governors--compare Cicero's almost certainly bogus accusations against Piso). It's hard to say whether Caesar's deal with Lucceius should count as a loan. Suetonius says that while Caesar was canvassing for the consulship:

Lucceium sibi adiunxit, pactus ut is, quoniam inferior gratia esset pecuniaque polleret, nummos de suo communi nomine per centurias pronuntiaret

He joined Lucceius to him [as opposed to Bibulus, who was Cato's man] so that he, seeing as that he was lesser in influence and worth more in money, would proclaim largess to the voting centuries in both their names, from his own funds

That seems to not technically be a loan (and in any case Lucceius didn't end up consul, Cato funded Bibulus with just as much money and got him elected alongside Caesar), and we have no more mention from either Suetonius or Plutarch that Caesar was still in debt after returning from Spain