This is just a reference. Use it to see how long will it take for you to get it done. The following is base on 9 returns with 6 en left over during the 16 hours play every day with regular regen speed (See this post.) The return means you burn your 42 en and return before en fills up completely. It doesn't necessarily means you have to follow the schedule to return 9 times, but minimum is 9 returns, can be more as long as you don't let en fill up completely. Again, this is just a reference. I'm not advising you to follow it. I'm just using it as an example.

During 16 hours, 9 returns with 6 en left over gives 42+42*9+6=426 en. And out of 426 en, you can do 213 waves(426/2=213) of portal. Lv1 incursion drops 2~4 essences, Lv2 drops 4~6, Lv3 drops 6~8 and Max drops 8~10. Since without gs, it's impossible to get to max incursion. I'll use average 4 essences per wave here. You get roughly 852 essences (213*4=852) per day.

Now, let's look at the amount of essences needed for each enchantment level:

Lv1: 100 essences

Lv2: 250 essences

Lv3: 500 essences

Lv4: 1,000 essences

Lv5: 2,500 essences

Lv6: 5,000 essences

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Total: 9,350 essences

It will take roughly 11 days (9,350/852=10.97) just to enchant one piece of gear from Lv1 to Lv6 provide that you always return before en fills up during 16 hours of play. If you are casual player, then estimate the amount of en you spend a day and you can calculate the time needed base on your play style. Say if you only play 1/2 of the time, 11 days become 22 days for one piece or 6 months for all 9 pieces. Say if you only play 1/3 of the time, 11 days become 33 days for one piece or one year for all 9 pieces. Of course, the math here is just an esimate. You may get more or less essences than the 4 average and also the luck enchant can play a very important role to speed up the process.

[Some interesting note]

Below is just pure assumption assuming 1 pure essence = 10 refined essences. Also assuming essences drop rate remains unchanged.

Lv1: 100 essences

Lv2: 250 essences

Lv3: 500 essences

Lv4: 1,000 essences

Lv5: 2,500 essences

Lv6: 5,000 essences

Lv7: 10,000 essences (assumption)

Lv8: 25,000 essences (assumption)

Lv9: 50,000 essences (assumption)

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Total: 94,350 essences (assumption)

110 days (94,350/852=110) needed to enchant one piece from Lv1 to Lv10.

990 days (110*9=990) needed to enchant all 9 pieces.