Bond Dissociation Temperatures for Hydrocarbons
When hydrocarbons undergo thermal decomposition carbon-hydrogen and carbon-carbon bonds are broken.
$$\ce{R-CH3 -&gt; R-CH2. + ~H.}$$
$$\ce{R-R' -&gt; R. + ~R'.}$$
Here is a link to a table that shows the energies required to break these bonds (the bond dissociation energy). These reactions are unimolecular decompositions and are governed by first order kinetics.  The rate of the reaction is given as
$\text{reaction rate} = k[\ce{A}]$
where k is the rate constant and $[\ce{A}]$ represents the concentration of the hydrocarbon.  The rate constant can be described using the Arrhenius equation
$k= A e^{-E_a/(R T)}$
where $A$ is the pre-exponential factor, T the absolute temperature in kelvin, ${E_a}$ is the activation energy, and R is the universal gas constant.  In the thermal unimolecular decomposition of a hydrocarbon, $A$ will have a value around $\pu{10^{12}sec^-1}$ and the bond dissociation energy can be used for the activation energy.
The Arrhenius equation tells us that this decomposition will be occurring at all temperatures above absolute zero.  As we increase or decrease the reaction temperature the rate constant will increase or decrease respectively, but the reaction is always occurring as long as we are above absolute zero.
As to pressure, yes it will affect the reaction rate.  Pressure is directly related to concentration, the $\ce{[A]}$ term in our rate equation.  As we increase or decrease the pressure, we are increasing or decreasing the concentration of $\ce{A}$, and thereby increasing or decreasing the rate of the reaction.