Electronic – Red resin in-between closely spaced PCB pads

I would say there are three major factors that would need to be considered before making a decision.

1. How many products are you planning on selling yearly? If it is a lot, say more than a hundred, you probably don't want to be doing much assembly. You need to save your time for future product development!
2. How many surface mount components are on the board and can you reduce that number more. Yes you can solder surface mount chips in your house and there are tons of cool methods for simplifying that process but still, nothing beats the speed and precision of a machine. (If you want some tips on how to solder surface mount chips ask that in another question. I have found some very good techniques) Anyways, thru-hole components are obviously way easier to solder by hand and if you are going to be doing more than a few boards and actually selling them to customers they are a lot easier do not screw up.
3. Do you enjoy circuit assembly. You need to consider what you like doing. If soldering boards all day gives you jollies and you have the time then maybe you should go for it. But if you are also interested in design and development of new stuff than you may not want to be tied down with your soldering iron all day.

Whatever you end up with, Good luck!

When soldering is done by hand there are lots of aspects that factor into how fast it can be done. Here are the largest factors for me when I am soldering:

1. The pitch of the components. I have this cover both IC pitch and 2 pin size. The smaller they are the slower the human has to go to get it set properly.
2. Through Hole components or SMD or both? Through hole components will actually take a lot more time to solder in my experience.
3. How closely spaced the components are. Humans will always make mistakes and the closer the parts are together the harder it will be to fix them. Also the closer they are together the more likely a slip of the hand will cause another component to get hit and have to be fixed.
4. How organized the parts and documents of assembly are. This one is pretty clear, if it is hard to figure what to put where it is going to take longer.

In general though, I would say it takes me about 45 seconds per part to place the parts on the board. The board can then be baking while more boards are assembled.

Where you have to be careful in time estimating is the time it will take to check for shorts and fix them. There will be errors, maybe just a couple out of the whole batch, but there will need to be more testing time when soldered by hand.