You're planning on adding more sugars to the brew, after it's completed fermenting? If not, then you're not actually doing a second fermentation. Most likely, you're talking about racking to another vessel, which is an old method. Many of us have stopped doing this for the vast majority of what we brew. For almost all of what I brew, which uses ale yeast, I simply leave the batch in primary/fermenter for 4-8 weeks, then move it to the kegs, seal, purge, and then chill and carbonate.
There are those that claim you need to do this to get a batch to clear properly. I've found it completely unnecessary. I pretty much always get a nice, compact yeast cake in the bottom of my fermenter. I'm using Wyeast strains from the British Isles, which I believe is a major factor there. Plus, I tend to use strains that are either highly or very highly flocculating.
Without an OG reading, you don't KNOW what you actually started at. If it's an extract kit, you should be within a few points of what the recipe calls out, provided the batch volume (in fermenter) was correct. You can check the SG after a couple of weeks, then taste the sample. Wait a few more days and do it again. IF the gravity reading is identical on both days, and it TASTES ready (no off flavors) then you can bottle it up.
Next time, and moving forward, take an OG reading so you know where you're starting. While this isn't critical with extract batches (still a good habit to form) it is very important once you go either partial mash or all grain. Many of us have switched to using a refractometer for the OG reading, then use the hydrometer for the FG reading. With the refractometer, you need a miniscule sample (a drop or two) to get the reading. You can also pull from the mash to see what you have there. A great thing when you're mashing the grain.
As for your batch, without knowing what you brewed, I'd let it go 2-4 weeks before taking a reading. Depending on how it tastes, you could let it go even longer. There are plenty of us that have left brews on the yeast for several weeks. Some have even left it there for months, with zero issues.
Also, 99% of the time, you can ignore the instructions where it lists how long to ferment, and then bottle. Unless you match the conditions perfectly, and pitch the correct amount of yeasts, the time frames listed are just a loose guide (at best). Once you've brewed enough batches, with the same yeasts, you'll get a better handle of how long they take to ferment certain OG range brews.
