How long would it take for a single laptop to mine a Bitcoin?

So you want one of this?

with a laptop?

For the sake of answer, let us assume you have a gaming laptop atleast (with a good graphics card). Let’s choose;

A laptop costing 1.2 Lakh INR or 1870 USD.

It has a 6 GB Nvidia 1060 graphics card with i7 7th gen processor and 16 GB of RAM. A top end laptop.

Currently,

1BTC = 7737 USD
Difficulty Factor of BTC is 2874.6742 G
Next difficulty retarget occurs at block 510047.0 (eta 13.4 days): 2.93031655023e+12 / +1.9% [est.]
BTC / Block: 12.50000000
Now hash rate of this laptop will be around 18- 20mh/s.

1 unit of electricity in India (Mumbai) is approximately 5–6 INR for the above 300 units slab

Electricity tariff in Maharashtra, Mumbai, Mahadiscom, Reliance Energy, BEST

So, 1 unit/ 1 kWH of power costs ~ 0.078 $

Enter these values here:

Bitcoin Mining Calculator

SO THE PART YOU HAVE BEEN WAITING FOR:

Your investment in the laptop - 1870 $
Now lets start mining day and night continuously 24x7x30. Remember mining is a very intensive process and its just like playing heavy games on your laptop all day and night. So you need a good air-conditioning and cooling for the laptop.
Let us calculate electricity costs. Compare the laptop with 200 watt electric bulb as the laptop will not draw more power than that.
Head to How much electricity does it use?
So the laptop alone will take around 33 units of electricity per week.
So in 1 week of mining; we incur 2.5 $ for electricity. So 2.5 USD/week for mining.
BTC earned in 1 week with laptop =0.00000001 BTC = 0.0001 USD (As per BTC mining calculator-link given above)
So in 100000000 weeks we get 1 BTC assuming the hashing difficulty remains same which is impossible as difficulty increases exponentially but for the sake of answer we will assume the difficulty remains constant
So to earn a Bitcoin with this 1.2 Lakh INR (1870 USD) laptop it will take 1917808.21 years or 19178 centuries with this configuration.
Now lets assume it takes 25 years to create a generation of family. So you will have 76712.32 generations before you can create one bitcoin with a laptop.
Now lets calculate the costs of creation of this 1 BTC; assuming an average laptop lasts for 4 years when kept running 24x7x365.
You need 479452 laptops for this purpose. Assuming no inflation and price of laptop remains same, that is 896575338.17 USD for laptop and 262079998.74 USD for electricity.
Adding them up, the costs for creating one bitcoin will be 1158655336.91 USD which is 1,158,655,336.91 = 1.1 billion USD.
So the cost of creation of BTC will be profitable only if the value of one BTC exceeds this.
I have purposefully ruled out the air-conditioning and rent costs to keep answer less complicated and simple.